2006) 249–263www.elsevier.com/locate/geomorph

Geomorphology 79 (

Human impacts on headwater fluvial systems in thenorthern and central Andes

Carol P. Harden

Department of Geography 304 Burchfiel Geography Building, University of Tennessee, Knoxville, TN, 37996-0925, USA

Received 25 August 2005; received in revised form 6 June 2006; accepted 6 June 2006Available online 17 August 2006

Abstract

South America delivers more freshwater runoff to the ocean per km2 land area than any other continent, and much of thatwater enters the fluvial system from headwaters in the Andes Mountains. This paper reviews ways in which human occupation ofhigh mountain landscapes in the Andes have affected the delivery of water and sediment to headwater river channels at local toregional scales for millennia, and provides special focus on the vulnerability of páramo soils to human impact. People haveintentionally altered the fluvial system by damming rivers at a few strategic locations, and more widely by withdrawing surfacewater, primarily for irrigation. Unintended changes brought about by human activities are even more widespread and includeforest clearance, agriculture, grazing, road construction, and urbanization, which increase rates of rainfall runoff and accelerateprocesses of water erosion. Some excavations deliver more sediment to river channels by destabilizing slopes and triggeringprocesses of mass-movement.

The northern and central Andes are more affected by human activity than most high mountain regions. The wetter northernAndes are also unusual for the very high water retention characteristics of páramo (high elevation grass and shrub) soils, whichcover most of the land above 3000 m. Páramo soils are important regulators of headwater hydrology, but human activities thatpromote vegetation loss and drying cause them to lose water storage capacity. New data from a case study in southern Ecuadorshow very low bulk densities (median 0.26 g cm−3), high organic matter contents (median 43%), and high water-holding capacities(12% to 86% volumetrically). These data document wetter soils under grass than under tree cover. Effects of human activity on thefluvial system are evident at local scales, but difficult to discern at broader scales in the regional context of geomorphic adjustmentto tectonic and volcanic processes.© 2006 Elsevier B.V. All rights reserved.

Keywords: Human impact; Soil erosion; Fluvial geomorphology; Soil moisture; Andes

1. Introduction

The Andes Mountains are the primary headwaterregion of the continent of South America, whichdelivers more freshwater runoff to the ocean per km2

land area than any other continent. Assessments of allnatural freshwater resources indicate that the natural

E-mail address: charden@utk.edu.

0169-555X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.geomorph.2006.06.021

internal freshwater resources of South America aresecond only to those of Asia (Table 1, FAO, 2003a).Rivers in the Andes, like rivers in mountain regionsacross the globe, respond to gradients, material prop-erties, and inputs of water and sediment. Unlike mostmountain drainage basins, however, those of the tropicalAndes have been population centers for millennia andare locations of major cities. The rich history of humanhabitation, especially in the northern Andes, and the

Table 1World water resources by continent (FAO, 2003a)

Continent Total water resourceskm3 yr−1

% of world freshwaterresources

Asia 12,461.0 28.5South America 12,380.0 28.3North and CentralAmerica

7443.1 17.0

Europe 6619.4 15.2Africa 3950.2 9.0Oceania 910.7 2.1World 43,764.3 100

250 C.P. Harden / Geomorphology 79 (2006) 249–263

rapid economic development of Andean countriesduring the past half-century make this an especiallyinteresting region in which to examine effects of humanactivities on fluvial geomorphology. Although physicalprocesses by which humans affect runoff, erosion, andsedimentation are not unique to the Andes, examinationof the intensity and variety of human uses of Andeanlandscapes offers a different perspective on human im-

Fig. 1. Map of the norther

pacts on fluvial systems from that obtained in the north-ern hemisphere.

Most (85%) of the continent of SouthAmerica drains tothe Atlantic Ocean. Brazil, the largest country, contains themost fresh water, but mean annual internal freshwaterresources per area are greater in Colombia (1.85 m/yr),Ecuador (1.52 m/yr), Peru (1.26 m/yr), and Venezuela(0.79 m/yr) than in Brazil (0.63 m/yr)(FAO, 2003a).Focusing on the five tropical countries of Venezuela,Colombia, Ecuador, Peru, and Bolivia that form theAndean headwaters of the Amazon (Fig. 1), this paperreviews deliberate and unintentional impacts of humanactivities on headwater rivers in the northern and centralAndes, and highlights human activities that increaseoverland rainfall runoff and cause more sediment toenter river channels. Zooming in from the spatial scales ofa region and countries to the scales of a watershed andsmall plots, it investigates the unusual characteristics andhydrologic importance of páramo soils of the northernAndes and comments on the relative importance of humanimpacts at different spatial scales.

n and central Andes.

Fig. 2. Exposure of black páramo soil, under tussock grasses andperched on a fine-textured deposit, much lighter in color, CajasNational Park, Ecuador.

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2. Regional setting

2.1. The physical setting

The cordilleras of the Andes divide the Pacific fromAtlantic drainages and capture and regulate the flow ofwater for most of the continent of South America. On theAtlantic side, the Amazon River basin comprises 34% ofthe land area of South America. Other major drainagebasins are theOrinoco (Atlantic),Magdelena (Caribbean),and Paraná (Atlantic). The highest peaks reach to nearly7000m above sea level. On the Atlantic and Pacific sides,relief is dramatic and hillslopes and river gradients aresteep. Westward-flowing rivers from southern Ecuador tocentral Chile deliver water to arid lands as they flow frommountain to ocean. Most of the region is tectonicallyactive, and sections of it are volcanically active.

Rainfall tends to increase with elevation. Moisturereaches the mountains primarily from the east, but rainfallin some InterAndean valleys and on the western flank ofthe cordillera is also influenced by weather originating inthe Pacific. ENSO effects are evident, although smalldifferences in the intensity or penetration of ENSO eventschange ENSO influences in the higher mountains (Tarras-Wahlberg and Lane, 2003). TheMagdelena River basin ofColombia has generally received less rain in El Niño yearsand more in the La Niña years (Restrepo and Kjerfve,2000). Similarly, pulses of deposition from Andean head-waters, recorded in floodplain sediment cores in the Beniand Mamore river basins, two Bolivian tributaries of theAmazon, were associated with La Niña events (Aaltoet al., 2003). On a longer timescale, sediment coresextracted from one high, Amazon-draining lake on thewestern cordillera in Ecuador showed periodic episodesof high sedimentation rates that correlated well withHolocene ENSO fluctuations (Rodbell et al., 1999).

The highest peaks are snow-capped. Below the snow-line (ca. 4800–5000 m in Ecuador and Colombia) andabove the upper limits of trees and most cultivated land(ca. 3000–3500 m), the northern Andes are characterizedby the páramo environment. The páramo ecosystem, and,in particular, páramo soil, is considered to be the principalregulator of the terrestrial hydrological system of thenorthern Andes (Podwojewski and Poulenard, 2000;Hofstede, 2001). These soils, in Ecuador, Colombia, andVenezuela, consist of a very black, highly organicepipedon (A, Ah, and/or O horizons) discontinuouslyoverlying an unrelated, inorganic surface (Fig. 2).Because mineral particles in páramo soils are eolian inorigin, páramo soils near to and downwind from activevolcanoes may be 1–2 m thick, while soils farther fromash sources or on glaciated surfaces may be only 20–

30 cm deep. Páramo vegetation varies, but is most com-monly grass (Calamagrostis sp., Stipa sp., and Agrostissp.), with some shrubs in less disturbed sites. Organicmatter decomposes very slowly in the moist, coolconditions that result from high elevation, frequentcloudiness, fog interception, and plentiful rainfall (ca.1000–2000mm yr−1). Typical mean annual temperaturesare 10–12 °C (Medina and Turcotte, 1999). Páramo soilsare classified as Andosols or Histosols, depending on theorganic matter content, which is typically around 30%.Low bulk-densities make them very sensitive to distur-bance from humans and livestock. As in the páramo, thescarcity of trees in the drier puna grasslands of the centralAndes is thought to result from anthropogenic burningand forest removal (Gade, 1999).

Because the Andes are still tectonically active (Nor-abuena et al., 1998), the physical setting includes activevolcanism, ongoing uplift, earthquakes, and high magni-tude mass movements. Uplift has caused rivers to incise(Safran et al., 2005) and denudation rates to be high (Aaltoet al., 2006). From Colombia to Bolivia, ten volcanoeshave erupted since 1964 (Table 2), and six others eruptedearlier in the 20th century. Volcanoes contribute sedimentto fluvial systems by direct input, by producing bare slopesvulnerable to erosion processes, by blanketing the sur-rounding landscape with ash, and by oversteepening

Table 2Volcanoes active since 1964

Country Volcano name Recent eruption

Bolivia/Chile Irruputuncu 1995Colombia Galeras 2002Colombia Nevado del Ruiz 1991Colombia Purace 1977Ecuador Guagua Pichincha 2004Ecuador Reventador 2003Ecuador Sangay 2004Ecuador Tungurahua 2004Peru Sabancaya 2003Peru Ubinas 1969

Source: Smithsonian Institution (2005).

Table 3Recent, major mass movements

Year Country Location Comments

1962 Peru NevadosHuascaran

13×106 m3, 400–500 personskilleda

1970 Peru NevadosHuascaran

30–50×106 m3, 18,000 killed;triggered by M 7.7 earthquakea

1974 Peru Mayanmarcarockslide

1.6×106 m3, created 150-m highdam on the Mantaro River, 450persons killed.b

1985 Colombia Nevado delRuiz lahars

90×106 m3, killed over 23,000in Armero;triggered by volcanic eruption.c

1987 Ecuador Reventadormassmovements

75–110×106 m3, ∼ 1000persons killed,earthquake-triggered rock and earthslides, debris avalanches, anddebris and mud flows; most deathsfrom floods.d

1993 Ecuador La Josefinarockslide

30×106 m3, N35 killed, debris damon Paute River.e

1994 Colombia Paezlandslides

Thousands of slides in 250 km2 area,earthquake triggered, 270 dead,1700 missing.a

a USGS, 2005.b Martinez et al., 1995.c Pierson et al., 1990.d Schuster et al., 1996.e Plaza-Nieto and Zevallos, 1994.

252 C.P. Harden / Geomorphology 79 (2006) 249–263

slopes, thus, making them vulnerable to landsliding. His-torical reports indicate that a 1773 eruption of TungurahuaVolcano (Ecuador) dammed the Pastaza River, and that thesame river had been dammed three times by eruptions in1918 (Zevallos, 1996). Ash, ice, and water in combinationcreate lahars, which fill and alter river channels for longdistances. Lahars produced by the 1985 eruption ofNevado del Ruiz volcano in Colombia moved a total ofabout 9×107 m3 of lahar slurry to areas up to 104 km fromthe source (Pierson et al., 1990). In Ecuador, a lahar froman 1877 eruption of Cotopaxi volcano followed a rivervalley to the Pacific Ocean, over 250 km away (Hall,1977).

Mass movements are important in delivering sedi-ment to the river channels of the Andes (Table 3). On theeast flanks of the Andes in Bolivia, where anthropogeniceffects on erosion processes appear to have beenminimal (Aalto et al., 2006), landsliding is widespread(Safran et al., 2005). In this region of steep slopes, massmovements are readily triggered by wet conditions andby earthquakes. The rugged topography favors land-sliding and the formation of landslide dams. In anassessment of mass movements (rock and earth slides,debris avalanches, debris and mud flows) triggered bytwo earthquakes about 25 km north of Reventador Vol-cano in northeastern Ecuador, Schuster et al. (1996)found the greatest amount of property destruction tohave been caused by flood surges of the main rivers,which had been near flood stage before large volumes oflandslide debris were added to them. The largest floodsurges were caused by the breaching of temporarylandslide debris dams. In the case of the large 1993 slopefailure at La Josefina (southern Ecuador), 30×106 m3 ofdebris filled the channel of the Paute River for 1 km ofits length (Plaza-Nieto and Zevallos, 1994). Theengineered but catastrophic release of the landslidedam 33 days later re-formed the channel downstream,

locally increasing the bed slope and causing rapid in-cision through the deposit.

2.2. Population trends

In the northern and central Andes, the cordillera of theAndes is not a single spine, but a region ofmultiple rangeswith elevated valleys and plateaus. In the north, changingpatterns of subduction of the Nazca plate under the SouthAmerican plate have built parallel ranges, with highInterAndean valleys between. This pattern is seen inColombia, where three parallel north–south-trendingranges define the landscape, and in much of Ecuador,where a relatively well-defined InterAndean valley isflanked by two parallel ranges. Farther south, the highestpeaks lie east of the Altiplano. In this tropical region,higher elevation valleys and plains have long been fa-vored for human settlement—daytime temperatures arepleasant, health-threatening insects and other diseasevectors are rare, and soils support productive agriculturewhere moisture is adequate. Today, the InterAndeanvalleys contain major towns and cities, including Bogota,Colombia (2640 m), and Quito, Ecuador (2850 m).Cusco, Peru is at 3250 m, and La Paz, Bolivia between3300 and 3600 m. Bogotá had 6.8 million inhabitants in

Table 5Dams by country, as reported by FAO (1998–2002)

Country Number of dams Reservoir storage

Bolivia 5 large ndColombia 26 large (N25×106 m3) 9.1 km3

90 medium and small 3.4 km3

Ecuador 12 7.5 km3

Peru N3 2.7 km3

Venezuela 96 157 km3

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2003, Quito had 1.3 million (in 2001), Cusco had over300,000 (in 2002), and La Paz had 789,585 (in 2001)(Citypopulation, 2006). Many contemporary settlementsoccupy sites with long histories of human use.

In spite of significant rates of international emigra-tion, populations of Andean countries continue to growand to become more urbanized (Table 4). Fertility ratesremain high by North American standards, ranging from2.6 (Colombia and Peru) to 3.9 (Bolivia), compared to1.9 (children per woman) in the United States (Earth-trends, 2003). Population increased dramatically be-tween 1950 and 2005 in the five countries of Bolivia,Columbia, Ecuador, Peru, and Venezuela, although notall of the increase was in the Andean highlands.

3. Deliberate human impacts on the Andean fluvialsystem

As populations have grown and economies devel-oped, countries have deliberately altered fluvial systemsto take advantage of water resources. The two mostsignificant intentional interventions are dams and waterwithdrawls. Major dams have been built for hydroelec-tric power generation and to store water for irrigation(Table 5). Abundant water and dramatic drops in ele-vation have allowed the northern Andean countries tobecome dependent on hydroelectric power. In Vene-zuela, 60% of the power is hydroelectrically generated(Earthtrends, 2003). At least 60% of the electric powerof Ecuador is generated at a single large dam, the DanielPalacios Dam on the Paute River (Hofstede, 2005). Bytrapping sediment and altering flow, dams profoundlyinfluence downstream reaches.

Water withdrawls for irrigation are more widespreadthan dams. In Bolivia, Ecuador, and Peru, more than 80%of surface water withdrawls are for agricultural irrigation

Table 4Population growth and urbanization by country

Country Population1 Annualpopulationgrowth1

Population% increase1

Percenturban2

%growth

2002 RuralAreas

UrbanAreas

1950–2002 1965 1989

Bolivia 8,705,000 0.3 3.8 221 40 51Colombia 43,495,000 0.0 2.7 246 54 69Ecuador 13,112,000 0.2 4.0 425 37 55Peru 26,523,000 0.7 2.6 248 52 70Venezuela 25,093,000 2.0 2.1 392 70 84

Source: 1Earthtrends, 2003, 2Valladares and Prates Coelho, 1993.

(Table 6). Some irrigation systems, such as those used inPeru by the Incas and in northern Ecuador by pre-Hispanic populations (Knapp, 1991), pre-date theSpanish conquest by at least hundreds of years. Theamount of water withdrawn for irrigation increaseddramatically in the second half of the 20th century as aresult of population growth, land reform, and govern-ment efforts to intensify agriculture. In Colombia, mod-ern projects for public irrigation were initiated in 1936,and, in Bolivia, a commission began planning for publicirrigation projects in 1938 (FAO, 1998–2002). Irrigationwithdrawls in the Andean countries expanded followingagrarian reform, which occurred from the 1950s (Bo-livia) through the late 1960s (Peru). Irrigation hastypically been small-scale (micro-irrigation), as land-holdings are small (b5–10 ha) (e.g., White and Mal-donado, 1991), and hundreds of irrigation districtsmanage the withdrawl and distribution of irrigationwater. Internationally available data on irrigation with-drawls, which aggregate data by country, reveal majorincreases in irrigation over recent decades, but do notseparate Andean data from that of lowland drainages.Abstraction of water for irrigation reduces in-streamflows, especially in drier seasons. The extent to whichabstraction affects geomorphically important flow levelsin the Andes remains unexamined. Water added to thelandscape by irrigation can reduce soil erosion by

Table 6Water withdrawls by country and sector (FAO, 2003a)

Country Internalrenewablesurfacewater km3

Totalannualwithdrawlas % ofrenewablewater

Withdrawls

Agriculture Domestic Industry

Bolivia 277 0.3% 87% 10% 3%Colombia 2112 0.5% 37% 59% 4%Ecuador 432 4.3% 82% 12% 6%Peru 1616 1.2% 86% 7% 7%Venezuela 700 0.8% 46% 44% 10%

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improving protective vegetative cover, but too muchwater added can destabilize slopes and water addedrapidly on steep slopes causes soil erosion.

4. Human activities that increase soil erosion andsediment in rivers

Sources of sediment in rivers include hillslopes andriver channel erosion. Sediment is added to the fluvialsystem by overland flow, mass movement, deliberatedumping, or anthropogenic reworking of sediment storedin channel beds. All of these occur in the Andes. Studiesacross the globe have linked increases in sediment yieldsof rivers with changes in land use (e.g., Ostry, 1982;Walling, 1983). Forest clearance for cultivation increasessediment yields by two to three orders of magnitude(Ongley, 1996). Agriculture can also be a primary sourceof eroded sediment (Bennett, 1939).

Irrigation increases river sediment loads, by reducingthe flow and corresponding dilution effect, and also byconveying water across steep slopes with high erosionpotential. Analysis of air photos from1976 and 1989 and afield survey in 1999 of a 900-ha catchment in the southernEcuadorian Andes found new gullies, which were attri-buted to poor construction and management of irrigationinfrastructure (Vanacker et al., 2003b). Gully formationwas observed to be a consequence of the spillover ofwaterfrom open canals and irrigation reservoirs and of mis-management of extra irrigation water. In a similar study ofa different Ecuadorian watershed, the proximity of gulliesto the river appeared to control differences in suspendedsediment concentrations between two subcatchments(Vanacker et al., 2003a).

Fig. 3. Homemade weir catches marketable sa

Mining activities contribute suspended sediment aswellas metal pollutants. Since the Spanish conquest, the Andeshas been known for deposits of gold, silver, and othervaluable metals. Gold mining remains active, and goldextraction more than doubled in Bolivia, Colombia, andVenezuela between 1950 and 1985 (UnitedNations, 1990).Gold production in the Portovelo–Zaruma mining districtof western Andean Ecuador increased 80% between 1994and 1999 (Tarras-Wahlberg and Lane, 2003). Copper, iron,lead, manganese, and zinc are also extracted in largequantities (United Nations, 1990). In 1991, a database ofgeological deposits in the Andes that have already beenmined, are currently being mined, or are under evaluationfor mining contained over 3300 records (BRGM, 2001).Although effects of mining on fluvial systems in the Andeshave been little studied, mines are typically located inremote areas and mining regulations not well enforced(Tarras-Wahlberg and Lane, 2003). Excavations on steepslopes send debris cascading into streams and also increasethe frequency of landslides. Failures of tailings impound-ment also send debris into rivers. In one study of the effectsof gold mining in the Puyango River basin on the westernflank of the Andes in Ecuador (Tarras-Wahlberg and Lane,2003), concentrations of suspended sediment derived frommining were apparent in drier years, but represented only asmall proportion of the estimated sediment yield in wetyears. Restrepo andKjerfve (2000) cited goldmining in theCauca basin of Colombia as an important contributor tohigh sediment concentrations in the Magdelena River,which has one of the highest sediment yields on thecontinent.

Another form of mining that has affected fluvial sys-tems is the in-stream removal of sand, gravel, and river

nd at high flow in a headwater stream.

Fig. 4. Soroche landslide in the upper Machángara River tributary ofthe Paute River in southern Ecuador.

255C.P. Harden / Geomorphology 79 (2006) 249–263

rocks for construction purposes (Fig. 3). In-stream ex-traction was widely practiced in Andean streams in Ec-uador in the early 1990s (Harden, 1993) but hasdiminished greatly because of recognition of the envi-ronmental consequences and enactment and enforcementof new regulations. In-stream mining suspends sedimentat all flow levels rather than only during runoff events. Inmined river systems, then, rainfall is not a good predictorof suspended sediment loads.

Steep gradients promote fluvial transport of sedi-ments. Dams and reservoirs trap sediment, calling at-tention to the magnitude of sediment loads whenreservoir storage capacity is lost. The Daniel Palaciosdam on the Paute River in Ecuador originally, in 1983,contained 120×106 m3 of storage capacity, but thatcapacity had been reduced to 95×106 m3 by 1993because of sedimentation. The mean annual rate ofsedimentation in the reservoir increased from2.47×106m3

to 2.76×106 m3 after May, 1993, when the release of thelarge landslide dam on the Paute River at La Josefinamobilized additional sediment (Zevallos and Jerves, 1996).An expensive dredging program has been used to maintainstorage because of the important hydroelectric power plantassociated with this dam.

Mass movement hazards are increasing globally aspopulation pressure and economic development pushpeople onto steeper lands and natural vegetation is re-moved for cultivation and other purposes (Vanackeret al., 2003c). Although landsliding is a natural adjust-ment to the crustal shortening occurring in the Andes, notall of the major Andean landslides have been completelynatural events. Two of 11 causal factors listed in ananalysis of the 1993 La Josefina rockslide were exca-vation at the toe and diversion of drainage for miningpurposes (Plaza-Nieto, 1996). Higher in the Paute Riverbasin, locals attribute the 2003 reactivation of a formerlandslide (Soroche) to alterations in agricultural drainageabove the failure (Fig. 4).

5. Land use effects on rainfall runoff

One of the most widespread human impacts to thefluvial system in theAndes, and in othermountain regions,is increasing the proportion of rainfall that reaches rivers assurface runoff. Changes in the rates of runoff-generationoccur as unintended consequences of nearly all humanactivities. Forest cover, agricultural practices, grazing,urbanization, and road construction have important andspatially extensive effects on runoff, which then controlsrates of erosion and sediment movement. These effects arenot unique to the Andes, but are especially interesting inthe Andes because of the intensity of human occupation

and the special natural and cultural characteristics of An-dean highland environments.

Forest clearance is part of the legacy of human oc-cupation of the Andean region. Contemporary deforesta-tion is occurring primarily in areas, generally on theexternal margins of the Andes, where forests remain, or indry climates, where tree removal promotes desertification(Ministerio de Desarrollo Sostenible y Medio Ambiente,1996). InterAndean valleys appear to have been clearedeven before the Spanish Conquest, although experts dis-agree about whether certain areas, which today aremarginally dry or have thin soils, ever supported trees(Acosta-Solis, 1977; Ellenburg, 1979; White, 1985;Gade, 1999; Sarmiento and Frolich, 2002). Photos takenin the late 19th and early 20th centuries in the province ofTungurahua, Ecuador show far less tree cover than existsthere today (Banco Central, 1984). The introduction ofEucalyptus trees in the 1860s led to the reforestation ofcleared areas and the present-day dominance of Eucalyp-tus throughout the InterAndean region (Dickinson, 1969).

Forest cover continues to be lost, even while refores-tation programs are adding trees. Between 1990 and 2000,Bolivia lost forest cover at an average rate of 0.3% peryear, Colombia at 0.4%, Ecuador at 1.2%, Peru at 0.4%,and Venezuela at 0.4% (FAO, 2003b). Much of this

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change occurred on external flanks of the Andes, andsome occurred in lowlands rather than in Andean regions.Studies of land-cover change have documented transi-tions from native trees to plantations of exotic species,principally Eucalyptus and Pinus, without changing theoverall extent of tree cover in the study area. Vanackeret al. (2003a) detected profound changes in land coverbetween 1962 and 1995 in the Machángara watershed insouthern Andean Ecuador, which included an overall gainin total wooded area even though 41% of the secondarynative woodland was cleared during that period. Theyattributed the changes to land reform programs and pop-ulation growth, and the net gain to reforestation withEucalyptus on formerly degraded lands. Gentry andLopez-Parodi (1980) linked deforestation in the upperAmazon watershed in Peru and Ecuador to increased highwater levels in the Amazon River at Iquitos and inPeruvian tributaries upstream of Iquitos.

The illicit drug trade, specifically of coca in Peru,Bolivia, andColombia, has been identified as a contributorto deforestation in the tropical Andes (U.S. Department ofState, 2001). Illicit coca production has typically involveda slash-and-burn approach in which fields are abandonedafter two to three growing seasons and new fields clearedto gain fertility and evade authorities. If growers havemoved onto the land from other environments and areunfamiliar with local conditions, coca cultivation is morelikely to promote runoff and soil loss. The U.S. StateDepartment (2001) estimated that a minimum of 2.4 Mhaof forest were cleared for coca production in the Andeanregion over the previous 20 yrs.

Globally, research has demonstrated that forest clear-ance leads to less rainfall interception, less infiltration, lessevapotranspiration, and more surface runoff (e.g., Boschand Hewlett, 1982). Loss of forest cover, thus, increasesstorm hydrograph volumes and shortens lag times to peakdischarges. Although forest removal typically increasesrunoff, reforestation does not necessarily reverse the trendand increase rainfall infiltration (Harden and Mathews,

Fig. 5. Increase of cattle in northern and central Andean countries.

2000). Soils are vulnerable to erosion following forestclearance, so clearedmountain slopesmay readily lose soil,and, thus, the ability to absorb rainwater, between the timesof clearing and reforestation. Many examples of degradedlocations, at which Eucalyptus trees planted for reforesta-tion out-competed the understory, exist in the region. Suchscenarios result in trunks of trees rising from bare surfaces,which continue to erode and degrade, and to which little orno new organic matter is added. Inbar and Llenera (2000)suggested that the massive reforestation of Eucalyptus inhighland Peru in 1976 was less effective than ancientterraces in preventing soil erosion.Where efforts have beenmade to replace páramo vegetation with pine trees toincrease carbon sequestration, pines have been observed toreduce water yields and dry the soils (Hofstede, 2001).

Other human activities that have altered rates andpatterns of rainfall infiltration and runoff are those thatcause soil compaction. Among these are the effects oflivestock grazing, increased tractor use in Andean agri-culture, growth of road networks, and urbanization. Graz-ing and trampling pressures increased greatly after theSpanish brought cattle, sheep, and horses to the Andes inthe early 1500s. Compared to the native camelids (llamas,alpacas, vicuñas), hoofed animals of European origin areheavier and exert more force per foot (White andMaldonado, 1991; Gade, 1999). When heavy animals,e.g., cattle, are confined to a limited area, their weightcompacts the underlying soil, and reduces infiltrationcapacity (Hofstede et al., 2002). The absence of a freezingwinter season means that trampled soils do not have anannual period of recuperation, so infiltration capacitiesremain low from year to year. Grazing has been locallyintense during the five centuries since Europeans arrived,and, where conditions have allowed, cattle numbers haveincreased in recent years (Fig. 5, FAO, 2006).

Contemporary agriculture can increase or decreaserates of soil infiltration and rainfall runoff. Althoughclearing forested land for cultivation usually has theeffect of increasing runoff, tilling cleared land increasesinfiltration capacities (Harden, 1991). The weight oftractors compacts the soil, so increased tractor use overthe past half-century (Fig. 6, FAO, 2006) can be expectedto have increased rates of runoff in Andean farmlands.The preferential generation of runoff on roads andfootpaths accelerates erosion on hillslopes (Harden,1992). The location of roads near streams allowsincreased runoff to discharge quickly from the land tothe fluvial system. Land reform programs in the 1960sand 1970s in Ecuador led to more land ownership ofsteeper hillsides and higher elevation fields. At the sametime, land became more parcelized (Vanacker et al.,2003a). Because boundaries between parcels interrupt

Fig. 6. Increase in tractor use since 1955, by country.

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overland flow, parcelization can reduce soil and waterloss from agricultural plots. The interrelated factors ofplot steepness and length, tillage practices, and thedistribution of croplands on hillsides make the hydro-logic effects of cultivation difficult to generalize.

Cessation of a land use that exacerbates runoff orsediment production does not necessarily mean return toa previous state of infiltration and sediment stability. Inthe dry central Andes of Peru, where annual rainfall isonly 350 mm, Inbar and Llenera (2000) found thatabandonment of agricultural terraces increases rates ofsoil erosion and sediment yields. More than 2 Mha ofhighland Peru are estimated to have been terraced, but91% of terraces in high areas have been abandoned(Inbar and Llenera, 2000). Plants do not grow naturallyon the terraced slopes in this dry environment. In morehumid Ecuador, where slopes were not terraced,previously cultivated sites that had been abandoned orleft in long-term fallow generated significantly morerunoff than sites under cultivation (Harden, 1996). De-graded soil, lack of moisture, and ongoing grazingstresses cause abandoned Andean farmlands to continueto be sources of runoff and sediment for many years(Harden, 2001). An additional factor that has reducedthe ability of the soil to absorb and hold rainwater hasbeen the wholesale removal of peaty soil. In Bolivian,turf has been sold as fuel (Godoy, 1990); in the puna ofPeru, peat soil has been mined for fuel and for horti-cultural purposes (Llerena, 1987); and, in Ecuador, thisauthor has observed peat mining for greenhouses thatproduce flowers for export.

6. Human impacts on the moisture storage capacityof páramo soil

Soils play a key role in determining whether rainfall isabsorbed by or shed from a site; they also store and releasewater. Human activities that cause soil compaction reducethe available pore space in the soil and thus reduce the

ability of the soil to store water. Soil moisture storagecapacity is also reduced when soil is lost or the organicmatter content of a soil decreases. The páramo soils,which cover 35,000 km2 of the northernAndes (Hofstede,2005), are especially vulnerable to changes that reducepore volume. With low bulk density and high organicmatter content, páramo soils are viewed as enormoussponges, which feed and regulate flow to the fluvialsystem (Luteyn, 2005). Water retention capacity ofundisturbed páramo soils is extremely high, reachingvalues of more than 100% at the wilting point, andstrongly correlated with the organic matter content(Buytaert et al., 2005; Luteyn, 2005). Poulenard et al.(2003) reported water contents in epipedons of páramoHydric Melanudands at 1500 kPa≥1000 g kg−1 andattributed high porosities to the abundance of organiccolloids.

Loss of vegetative cover promotes drying, whichirreversibly reduces pore space and the water-holdingcapacity of the páramo soil (Poulenard et al., 2003).Studies have shown páramo soils to become crusted andeven hydrophobic following disturbance (Poulenardet al., 2001). Fire is the principal human disturbanceaffecting the hydrology of páramo environments. Today,páramos are primarily burned to remove old grass andpromote the growth of tender new shoots as food forcattle. Gade (1999) reported that any grassy site in thehigh Andes is probably burned at least once each 5 yrs.Similarly, Hofstede (2005) suggested that only the mostremote or most protected páramo sites are not affectedby livestock. Páramos are also burned to clear land forcultivation, improve hunting, or implement local beliefsystems (e.g., bring rain, deter evil spirits) (Hofstede,2001). Grass páramos have been used as grazing lands atleast since the arrival of cattle, sheep and horses with theSpanish in the 1500s, so widespread burning, coupledwith trampling and vegetation removal by grazing,would have reduced moisture storage and increasedrates of runoff in páramo environments over the last fivecenturies. These practices have been so widespread thatthe region lacks control sites for comparative studies.

7. Case study of soil moisture

To more closely examine the soil moisture conditionsin the headwater region and to investigate differences insoil moisture between grass- and tree-covered highlandsites, a study was conducted of surface soil moisturecharacteristics in and near the 50 km2 Llaviucuwatershed,in the western cordillera of the Andes near Cuenca,Ecuador. The Llaviucu watershed is of special interestbecause it yields 20–30% of the water used by the city of

258 C.P. Harden / Geomorphology 79 (2006) 249–263

Cuenca and because most of it lies within Cajas NationalPark (Fig. 7). Elevations in the watershed range from3000 to 4300 m. Vegetation is predominantly grasspáramo, with occasional small stands of Polylepis trees athigh elevations (Fig. 8), tropical montane forest vegeta-tion below 3400mon the steep flanks of the glacial troughof the lower watershed, and imported pasture grasses onthe trough floor. The national park is managed by theMunicipality of Cuenca through a corporation with majorleadership by ETAPA (Empresa Pública Municipal deTelecomunicaciones, Agua Potable y Alcantarillado ySaneamiento de Cuenca), the local utility company. Inaddition to the usual goals of protecting the natural envi-ronment and fostering tourism, recreation, and environ-mental education, this park is also managed to maximizewater storage and dry season river flow. Cattle grazing inthe park has successfully been reduced from tens ofthousands to a very small number, and the corporationmust now decide whether to continue to burn the páramoregularly or allow vegetative succession to occur. Evi-dence in the region indicates that trees, which presentlyoccur in isolated islands up to 4300 m (Gade, 1999), maybecome dominant in the absence of burning (White andMaldonado, 1991), but the water resources effects of sucha change are not known.

Fig. 7. Location of Cajas N

For this study, pairs of soil moisture study plots wereestablished at elevations between 3163 m and 3527 m tocompare the effects of grass and tree cover where otherfactors–elevation, location, slope, parent material, soildevelopment–were the same. A HydroSense (CampbellScientific) Time Domain Reflectometer (TDR) wasdeployed in 4 m2 plots at 23 sites to obtain 5–15 repli-cations of in situ measurements of volumetric moisturecontent (VMC) at each site. The 12-cm TDR probes in-tegrated VMC along their length. In each plot, a 12-cmdeep soil sample was collected between one set of TDRprobe holes for laboratory determination of gravimetricmoisture content (GMC), and a second sample (0–12 cmdeep), taken within 1 m of the first, was extracted for bulkdensity determination. The sand replacement method wasused in the field to determine in situ volume for bulkdensity calculations. All soil samples were air-dried andthen oven-dried (24 h at 105–110 °C) and weighed. GMCsamples were weighed before drying, so that GMC couldbe calculated as the ratio of the mass of water (mwater=wetsoil mass−oven-dried soil mass) to themass of oven-driedsoil (GMC=mwater /msoil).

Loss-on-Ignition (LOI), interpreted as an approximatemeasure of the organic material (% by mass), wasdetermined as the percentage of the sample mass

ational Park, Ecuador.

Fig. 8. Páramo grassland in Cajas National Park, Ecuador.

259C.P. Harden / Geomorphology 79 (2006) 249–263

remaining after burning the oven-dried sample at 550 °Cfor 2 h. All sampling was done in June and July of 2004, arelatively moist year.

Volumetric moisture content is the ratio of the volumeof water to the volume of soil. As volume is the ratio ofmass to density, VMC may also be expressed as

VMC ¼ ðmwater=qwaterÞ=ðmsoil=qsoilÞ: ð1Þ

Using soil bulk density as ρsoil, and setting the densityof water (ρwater) at 1 g ml−1, the three values for VMC,GMC and bulk density at each site can be related as

GMC ¼ ðVMC⁎1 g ml�1Þ=ð bulk densityÞ: ð2Þ

The resulting data show extremely low bulk densities(median 0.26 g cm−3), high organic matter contents ofsurface horizons (median 43%), and high water-holdingcapacities (median GMC 1.52 g g−1; VMC ranges from12% to 86%). In other words, the mass of water in thesesoils is typically 1.5 times the dry mass of solid soilmaterial. Measurements from paired plots showed thatsoils under grass cover consistently had higher VMCthan soils under trees (Table 7), and field observationsshowed that soils under trees contained more macro-pores compared to soils under grass cover. In all pairs,soil under tree cover was less dense and drier than soilunder grass. A comparison of soil properties at fourpáramo soils sites, three that had burned recently andone that had not, showed little or no difference in bulkdensity or VMC between the four (Table 8).

Although bulk densities in the Llaviucu watershedstudy were very low, generally good agreement (sameorder of magnitude, most within 50%) between mea-sured and calculated values of soil moisture and fieldobservation provided confidence in the results. Thiscomparison between grass- and tree-covered soils waslimited by the small size of the database and by thedifficulty of finding sites at which all other factors wereequal, as trees were observed to typically occupy steeperand rockier sites. These initial results suggest that tree-covered sites store less moisture than grass-covered sitesand raise the question of whether páramo soils onlybehave as “sponges” under grass cover, which may, inturn, be an artifact of anthropogenic fire management.Lightning-caused fires have not been reported inpáramos, although lightning strikes have occasionallybeen observed (Luteyn, 2005).

The lack of difference in soil moisture and soil bulkdensity between recently burned páramo sites and anunburned, but similarly grassy site was not unexpected,as the longer-term history of all grass páramo sitesappears to include relatively frequent fires. Results fromthis study suggest that, not the fire, but the establishmentof woody plants in the absence of fire would increaseevapotranspiration rates and soil drainage (throughmacropore formation), causing páramo soil to lose itscapacity as sponge. A small sample can only be illus-trative, however.

A further confounding factor in the moisture storagecapacity of páramo soils is the depth of soil at a givenlocation. Soil in much of the Llaviucu watershed is thin

Table 7Data from paired plots in Llaviucu watershed, Ecuador

Vegetation Altitude(m)

Moisture GMC0–12 cm g g−1

Bulk density 0–12 cm g cc−1

Moisture VMCaverage 0–12 cm%

VMC range ofobs values %

VMC ratiograss:wooded

GMC g g−1

calculated asVMC/BD

GMC ratioof obs/calc

PAIR 1 2.8Grass 1 3170 1.52 0.20 68 48–76 3.4 0.4Grass 2 3170 1.80 0.34 65 53–79 1.9 0.9Wood 1 3175 1.06 0.04 20 8–34 5.0 0.2Wood 2 3175 nd nd 27 13–34

PAIR 2 2.8Grass 3 3248 0.49 0.54 35 18–57 0.6 0.8Grass 4 3248 0.70 0.48 0.7 1.0Wood 3 3260 0.73 0.09 12 8–24 1.3 0.5Wood 4 3260 0.89 0.11 1.1 0.8

PAIR 3 4.0Grass 5 3170 1.45 0.32 71 64–76 2.2 0.7Wood 5 3176 1.31 0.06 20 6–28 3.3 0.4Wood 6 3176 2.50 0.05 15 10–27 3.0 0.8

PAIR 4 1.3Grass 6 3163 1.45 1.08 60 53–65 0.6 2.6Wood 7 3273 1.10 1.06 47 38–58 0.4 2.5

260 C.P. Harden / Geomorphology 79 (2006) 249–263

(20–30 cm deep), compared to soils at other highelevation páramo sites less affected by glaciers.Although it is presently below the lower limit ofpermanent ice, the upper Llaviucu watershed wasunder part of a larger ice cap (Rodbell et al., 2002),and the lower Llaviucu watershed is the trough-shapedvalley of an outlet glacier. Field observations made in theLlaviucu catchment documented the flashiness of runoffon thin, quickly saturated soils in this glacially scouredvalley. The flashiness of runoff in the watershed andrelative dryness of its wooded soils, even in a wet time,can be readily explained, but are contrary to conven-tional expectations. This underscores the importance oflocalized differences and the need for more such casestudies, which should help management make informeddecisions for this national park in its effort to sustain and

Table 8Bulk density, Loss-On-Ignition (LOI), volumetric soil moisture(VMC) and gravimetric soil moisture (GMC) of páramo soils at foursites in Cajas National Park, southern Ecuador

Vegetation Elev.m

BulkDensity gcc−1

LOI%

VMCavg.%

VMCmin.%

VMCmax.%

GMCg g−1

Páramo 3465 0.47 29.3 74 72 76 153burned 1 32.5

Páramo 3467 0.37 42.2 80 78 82 211burned 2 48.5

Páramo 3469 0.28 54.5 84 83 85 238burned 3 58.5

Páramo 3527 0.27 57.6 86 84 88 252unburned

improve downstream water resources as well as betterunderstand the páramo in other areas.

8. Summary and conclusions

Ways in which Andean people and their activitiesaffect the flow and sediment loads of mountain riversare essentially the same as on other continents. Thetropical location of the northern Andes, belief systemsthat motivate certain land-management strategies, andthe dominance of grass páramo distinguish the Andesfrom other steep, volcanically active mountain regionsof the world. Beyond deliberately engineered changesof river impoundments and water withdrawls, humanactivities have many more subtle and unintentionalgeomorphic effects on the fluvial system. Forestclearance, grazing, agriculture, roadways, and urbani-zation increase the proportion of rainfall that flows tothe channel network during storms, and steep slopesgive overland flow the energy to erode and move finesediments. Human interventions that destabilize slopes,from mining, to tree removal, to problems withirrigation canals, contribute to triggering mass move-ments, the largest of which dam rivers and subsequentlyserve as fluvial sediment sources. Páramo soils, whichare hydrologically important in the northern Andes, areparticularly sensitive to drying, which occurs whenvegetation is removed by burning, grazing, or tilling.The Llaviucu case study verified the low bulk densityand high moisture-holding capacities of páramo andhighland soils and showed that soils under grass cover

261C.P. Harden / Geomorphology 79 (2006) 249–263

hold more moisture in a moist season than soils undertrees.

At the regional scale, natural processes of uplift anddenudation, volcanic activity, steep slopes, and massmovement create a geomorphically active environmentin the northern and central Andes. Where human settle-ment is sparse and relief is high, such as on the externalflanks of the Andes, the impacts of human activities arenegligible compared to the magnitudes of natural pro-cesses and adjustments in the fluvial system. A recentstudy in the Upper Beni River basin in the BolivianAndes found no significant change in rates of erosion,determined from 10Be analysis of quartz grains, over thelast millions of years (Safran et al., 2005). Likewise, abroader study of rates of erosion from 47 rapidly erodingdrainage basins in the Bolivian Andes concluded thatanthropogenic disturbance was minimal (Aalto et al.,2006). Given the great distance and the number ofsediment sinks in the Amazon basin between the Andesand the Atlantic Ocean, it is unlikely that human impactsin the Andean headwaters are noticed at the mouth of theAmazon.

On the scale of 103 km2 in human-dominated land-scapes of the InterAndean valleys and on a temporal scaleof years to centuries, unintended human impacts on soilerosion and runoff are evident, if not well documented.These are visible as truncated soils, reservoir sedimenta-tion, stream incision, increased duration of stream tur-bidity, and accelerated rates of mass movement wherepeople have steepened slopes through construction andmining. At the scale of plots (10m2) definite differences insoil properties are associatedwith differences in land coverand land use.

Many Andean river valleys are narrow and steep, sochanges in slope stability or changes that cause soil toerode or rainfall to run off are rapidly transmitted to theriver system. The land uses today follow a legacy, begunlong before the Colonial era, of forest clearance, agri-culture, and urbanization in the Andean region. Land useshave changed as populations have increased, local andglobal economies and technologies have changed, andland reforms have been implemented. Human impacts inthese mountains may now be more intensive andextensive, but they are not new.

Acknowledgements

The author completed the case study in CajasNational Park with the support from the FulbrightCommission, U.S. Department of State, with collabora-tion of faculty and students from the University ofCuenca, Ecuador, and with permission from Cajas

National Park, ETAPA, and the Municipalidad deCuenca. Additional thanks go to Alan Moore andMarisa Ernst for assistance in the field and to WillFontanez and David Moore for help with databases andgraphics.

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