Water ResourcesLecture 4

The River Environment (2)

Problems with generic zoning of a river:

• Many rivers are different – they occur in different geological and climatic regions.

• Different rivers have different transitions along their profiles.

• Rivers do not often undergo a gradual transition along their profile:– What about rejuvination at a nick-point?– What happens when a tributary with very different

characteristics joins the main stream?– What happens when a river is dammed along its course?

Hiererarchical Classification of River Systems

• Proposed by Frissel et al, 1986.• Adapted to a geomorphological classification for

Southern Africa by Rowntree and Wadeson, 1999. • Basic Principle:

– River System– River Zones– Segments– Reaches– Morphological Units– Hydraulic Biotopes

Frissel et al

Definitions (Frissel)

• Stream System:– All surface waters in a Watershed (catchment).

• Segment System:– Portion of a stream system flowing through a

single bedrock type, and bounded by tributary junctions or major waterfalls.

Definitions: Frissel

• Reach System:– A length of stream segment lying between

breaks in a channel slope, local side-slopes, valley floor width, riparian vegetation, and bank material.

• Pool or riffle system:– A subsystem of a reach having characteristic

bed topography, water surface, slope, depth and velocity patterns

Classification of Rowntree and Wadeson

Hydraulic Biotopes

• A mosaic of small patches of uniform condition within the wetted perimeter of a stream.

• Delineated by discontinuities in either flow or substratum.

Hydraulic BiotopesSubstrate Types• Bedrock• Boulder• Large Cobble• Pebble• Gravel• Sand • Silt/Mud

Flow Types• No flow• Barely perceptible flow• Smooth boundary turbulent• Ripple surface• Slow and fast riffle flow• Undular standing wave• Broken standing wave• Cascade• Chute• Free fall• Boil

The Rive Continuum Concept (Vannote et al, 1980)

• 4 dimensions to change in a river system.• RCC Deals with longitudinal change: Basic Principle:

Rivers possess a continuous gradient of physical and chemical conditions that are progressively and continuously modified modified downstream.

Driving variablesResponse variables

From: Davies and Day, 1998.

RCC

• Essentially all components of a river, at any point along its length, are dictated by abiotic and chemical conditions and modified by gradients of biological variables and processes that occur upstream of the point of consideration.

Criticism of RCC:

It does not focus on all 4 dimensions of change.

E.g: Junk et al, 1989: Flood Pulse Concept.

From Davies and Day 1998

Readings:

• Davies and Day, 1988• Frissel et al, 1986• Rowntree and Wadeson 1999• Vannote et al, 1980• Junk et al, 1989