MISSISSIPPI RIVER DIVERSIONSAND COASTAL RESTORATION

Christopher M. SwarzenskiUnited States Geological Survey, La Water Science Center, Baton Rouge

SOME CONTEXT

SOIL IS THE PRIMARY METRIC TO EVALUATE EFFECTIVENESS OF ANY RESTORATION APPROACH

- Quality: resists tendency to erode, such as from tides and storm surge

- Quantity: accretes vertically fast enough to keep pace with relative sea level rise (subsidence+eustatic sea level rise)

NUTRIENT REQUIREMENTS OF COASTAL MARSHES

NO peer-reviewed science that an insufficient supply of nutrients has contributed to wetland loss in coastal Louisiana or anywhere else in the world

NO peer-reviewed science that adding nutrients helps or has helped reduce rates of marsh loss in coastal Louisiana or anywhere else in the world

CHENIERPLAIN

MISSISSIPPI DELTAPLAIN

ActiveAtchafalaya River

ActiveBird’s foot DeltaInactive

New Orleans

WITH MINERAL MATERIAL,Recently deposited mudWest Bay, August 2011inorganic, heavy, inanimate

WITH ORGANIC MATERIALEroding shoreline,

Lake Salvadorhighly organic, light, “living”

Building and Maintaining Delta Wetlands

ACTIVE DEPOSITION

IN SITU ACCRETION

TYPES OF RIVER DIVERSIONS

I. WATER-QUALITY

a. “rejuvenation” e.g. Caernarvonb. “maintenance” e.g. Davis Pond

examples Caernarvon, Davis Pond, Terrebonne basin

II. MINERAL SEDIMENT (land-building)

examples include Wax Lake Outlet, West Bay, Bird’s Foot delta

“REJUVENATION”

PurposeTransform a degraded (marsh) soil into something less degraded => higher-quality soil

Constituents in River Water- fresh water (reduces salinity)- fertilizers (nitrate-N, orthophosphate) - sulfate- herbicides- but very few clays and silts

What we might hope for when we “rejuvenate” a marsh

Water-quality diversion

Existing degraded S. patens marsh soil

Soil of S. falcata marsh, typical of new salinity regime

“The diversion isn't filling the marsh with sediments on a grand scale”. But the effect of the added river water—loaded as it is with fertilizer from farm runoff—is plain to see. "It turns wetlands hanging on by the fingernails into something quite lush"

“Those nutrients are necessary for nourishing the marsh environment”

SCIENTIFIC CONSENSUS

“Two diversions on the Mississippi are already demonstrating how well diversions can work to restore wetlands. Although these are freshwater diversions (meaning they pull water off the top of the river rather than the bottom, where heavier sediment loads are) and thus do not provide a lot of sediment to the wetlands, they are still beneficial”

REALITY: Water quality diversions appear to compromise soils

I. Swarzenski et al 2005, 2008, freshwater Panicum marshesChronic passive inflow of Lower Atchafalaya River water since 1950s coincided with massive conversion of freshwater Panicum marsh to open water and degraded marsh, soils more decomposed than reference marshes, observation, linked to river water influx Biogeochemistry 90, p. 49-63.

II. Kearney et al. 2011 brackish S.patens marshesCaernarvon, White Ditch and Pte a la Hache Post Katrina, areas within outfall were more eroded than adjacent reference marshes at all three controlled diversions; no data on causes, remote sensing, correlation Geophysical Research Letters 38 L16405

III. Deegan et al, 2012, New England S. alterniflora salt marsh9 years of nutrient additions: plants produced fewer roots, organic matter decomposed more rapidly and creek banks collapsed, experimental, causative Nature 490, p 388-392.

I. Demise of Penchant Panicum marshes (1955-1998)

pre 1955 1998

• Rain is primary sourceof fresh water• Flow east to west• Solid expanse ofP. hemitomon

• River is primary sourceof fresh water• Flow west to east• Loss of > 60% of P. hemitomon marsh

Swarzenski et al Biogeochemistry 2008

Three to six months of passive river water influx for 40+ years (Lower Atchafalaya River example)

Soil Quality: with long-term (40+ years) river water influx decomposition appears enhanced (same plant community, Panicum hemitomon)

- No difference in year-end standing biomass- No difference in accretion rates-More organic matter in marsh receiving river water

BUT:- Soil is much more decomposed

- links to river water include sulfate, nitrate, alkalinity, pH

River water subsidyReference

Swarzenski et al Biogeochemistry 2008

OBJECTION: NUTRIENTS GOOD NUTRIA BAD

Nutria are the problem, herbivory is stimulated by nutrient enriched plant material, nutria multiply out of control and marsh dies off

Photo from Holm et al 2011, LPBF

River Reference/Rain

SEPTEMBER ABOVE-GROUNDSTANDING BIOMASS (n=4)

grams m2

504(30) Total 538(31)

337(27) Panicum 441(31)

SOIL (n=3)

14.5 12.8

225 187

Stock (kg C,.5 m by 1 by 1 m)

Accretion, (g CPer m2 per year)

Swarzenski et al Biogeochemistry 2008

Outfall

Hydrologicbarrier

Caernarvon

II. Controlled diversion at Caernarvon, post Katrina

Kearney et al 2011, Geophysical Letters

Outfall Marsh

East Controlin direct path of Katrina

MARSHES POST KATRINA WITHIN OUTFALL AND ON EAST SIDE OF DELACROIX RD

Kearney et al 2011, Geophysical Letters

OBJECTION: Pattern of loss is random, or energy was concentrated on west side, but not on east side of Delacroix Road

PRE KATRINA

POST KATRINA

Kearney et al 2011, Geophysical Letters

“ By contrast, when nutrients were added, the authors observed that plants produced fewer roots, organic matter decomposed more rapidly and creek banks collapsed, leading to wider creeks and less vegetated marsh, with narrower bands of S.alterniflora”

III. DEEGAN ET AL, 2012, Nature

OBJECTION:

Deegan et al 2012, Nature

New England salt marshes are different

- Macrotidal environment (5-7 ft tides)

- Ice formation (erosion)

1. WEAKENED SOILS ARE THE COMMON THREAD

- Correlational (field observations, remote sensing) and experimental studies in different marsh types from Atlantic and Gulf Coasts point to similar outcomes, weakening of soils: in areas with nutrient enrichment, soils are more decomposed and erode more readily during daily tides and during storm events

- excessive flooding could also contribute to weakening of soil

2. ABSENCE OF EVEN A SINGLE STUDY SUGGESTING SOIL IS GAINING SHEAR STRENGTH WITH RIVER WATER SUBSIDIES

Conclusions (Water-quality diversions)

TYPES OF RIVER DIVERSIONS

I. WATER-QUALITY

a. “rejuvenation”b. salinity control

examples Caernarvon, Davis Pond, Terrebonne basin

II. MINERAL SEDIMENT (land-building)

examples include Wax Lake Outlet, West Bay, Bird’s Foot delta

Scientific consensus is that massive sediment diversions off of mainstem river, in mid-Barataria Basin, are key to restoring coast

What is a massive sediment diversion?

50,000-80,000 cfs controlled ? 1-2 months per year?? 3-4 months per year??

>250,000 cfs uncontrolled??

Is mid-Barataria Basin a suitable location for massive sediment diversions in terms of initiating processes for a sustainable coast?

EVIDENCE THAT SEDIMENT DIVERSIONS OFF THE MAIN STEM WILL WORK: WAX LAKE OUTLET

"To say that there's not thriving vegetation there with the exact same water that's in the Mississippi River is just a fundamentally flawed statement,"

NAS, NWF, EDF, http://www.youtube.com/h?v=i-sJFoM-Lsc

Wax Lake Delta: a diversion that works

accretion (cm/year)

Min

eral

sed

imen

ts,

kgin

top

24

cm p

er m

2

0 1 2 3 4 50

50

100

150

200

250

Birds Foot

LAR/WLO

Significantly more mineral sediment is deposited in the Bird’s foot delta wetlands than in the Wax Lake/Lower Atchafalaya River wetlands

THE BIRD’S FOOT DELTA IS NOT BUILDING LAND

WAX LAKE OUTLET IS NOT A GOOD ANALOG FOR SEDIMENT DIVERSIONS OFF OF THE MAIN STEM MISSISSIPPI RIVER

SYNERGY Combined 1/3 of the Mississippi River flow and about ¼ of the sediment load

PLATFORM previous delta building occurred several thousand years ago, many years of marine reworking has built a firm platform to support weight of new mineral deposits

SUBSIDENCE AND COMPACTIONRates are probably much lower than in Barataria and Breton; Holocene layer is much thicker in latter area

Conclusions

I. WATER-QUALITY

a. as “rejuvenation” problematic, appears to exacerbate marsh loss; adaptive management calls for refinement of approach, detailed studies before being implementedb. as “maintenance” works in short term, but only needed occasionally

II. MINERAL SEDIMENT (land-building)

- specifics are needed: size, location, key uncertainty is a better understanding of weight bearing capacity of underlying strata

SHOULD DIVERSIONS PLAY A ROLE IN COASTAL RESTORATION

Emphatic yes, QUESTION IS HOW?

Clarity is needed on what the expectations are for main-stem river diversions as proposed in Master Plan and then a transparent scientific debate whether these expectations are reasonable given our geological and ecological knowledge.