When discussing water rights issues, it is crucial to have a general understanding of how fluvial systems function. This post will shed light on sediment movement and river system responses due to changes in watershed, river channel, and sediment supply.
Generally, a landscape’s topography is shaped by water that flows from high to low elevation. The potential energy supplies provided by these movements are expressed as channel and valley forms alongside aquatic and riparian habitats. Surplus of energy is dispersed amongst bank vegetation, turbulence and riffles in stream profiles, erosion at bends, channel bed roughness, and ice/sediment transport (Kondolf, 479).
Sediments are influenced by a variety of watershed components such as soil, vegetation & coverage type, land use, climate, weathering, and erosion rates. Watersheds have specific zones where these sediments are made, moved, and stored. The following diagram illustrates these source, transport, and deposition environments:
1. Source streams are where non-alluvial (clay, silt, sand, or gravel) enter from large and infrequent debris flows such as landslides and wasting failures.
2. Transfer streams are much sturdier conduits that have a high sediment transport capacity. They have the ability to move limited increases in loads of sediments and hardly changes in response to reductions in sediment supply.
3. Response streams are depositional environments where changes in sediment supply result in persistent channel instability.
River sediments are influenced by both watershed zones and local conditions of the river such as transport capacity, velocity, and depth. Sediment transport capacity is the amount and size of sediment that a river is able to move. This depends on the velocity and depth of the river, which is dictated by channel slope, discharge (volume of flow), and roughness of the channel (Marshak, 589). If any changes arise in the factors mentioned above, there will be changes in sediment transport capacity, which results in river and drainage network formations.
Additionally, there are three types of sediment load that dictates channel formation. First, dissolved load are solutes from chemically weathered bedrock and soils. Suspended loads include fine sands, clay, and silt that are often held in the water column due to turbulence. Finally, bed load carries cobbles, boulders, gravel, and sand, which are the most influential factor in river morphology (Morisawa, 1968).
With this, we can gauge the health of the river since there are general assumptions that can be made by comparing suspended loads and transport capacity. Such measurements indicate whether a river will erode more sediment, deposit extra sediment, obtain river equilibrium (optimum):
capacity > load = erosion
capacity < load = deposition
capacity = load = no net erosion/deposition
When sediment, hydrological load, and channel slope changes, the fluvial system adjusts to these conditions as a means to restore equilibrium. Channel evolution models help illustrate river responses to change and further shows that rivers are not static systems (Morisawa, 1968):
In conclusion, this post was meant to inform the reader that rivers are dynamic systems that are sensitive to change and will adapt to those condition changes. Management, commercialization, litigation, and mitigation strategies must consider short and long run effects when addressing rivers. The river’s morphology may be used as an indicator of ecosystem health and function. By applying this basic understanding of fluvial systems, one can operate more efficiently with rivers by acknowledging them as a fluent system rather than a site for collective problems.
Downs PW, and GM Kondolf. 2002. “Post-project appraisals in adaptive management of river channel restoration”. Environmental Management. 29 (4): 477-96.
Marshak, Stephen, and Donald R. Prothero. 2001. Earth. New York: Norton.
Morisawa, Marie. 1968. Streams; their dynamics and morphology. New York: McGraw-Hill.