Geomorphology: Fluvial Geomorphology
Fluvial geomorphology focuses on the dramatic hydrodynamic forces that shape rivers; the result of the interplay between the force of moving water and the materials forming the streambed. Hydrodynamic forces form islands and sandbars, grade rapids and riffles, create deltas, and form—and eliminate—meanders. Therefore, rivers and streams are not only conduits of water, but also of sediment. The water, as it flows over the channel bed, is able to mobilize sediment and transport it downstream. As water moves through a meandering channel, changes in streamflow cause the sediment to be scoured, sorted, and deposited in riffles and pools (MN DNR, Healthy Rivers).
There are eight variables determined by climate and geology that interact to create the form of a stream channel (Leopold et al. 1964; Heede 1992; Leopold 1994):
- Discharge (the volume of water)
- Sediment supply
- Sediment size
- Channel width
- Channel depth
- Water velocity
- Slope (or gradient)
- Roughness of channel materials
These variables reflect the importance of interactions between the stream and the watershed landscape of bedrock, topography, soils and climate within which it lies. These variables interact in predictable and measurable ways. For example, the natural form of the low gradient streams characteristic of most of Minnesota is sinuous, narrow, and deep. Steeper rivers found on the North Shore of Lake Superior are less sinuous and have boulder and bedrock rapids
The Wild Rice River downstream of Heiberg Dam in northwestern Minnesota, prior to 2005, eroded its stream banks, a result of the characteristically sediment-hungry water below impoundments. Colder water has an increased capacity to carry sediment.
Healthy streams are able to carry a certain amount of sediment over time in a sustainable balance. With the addition of excessive sediment (soil erosion from farmland, for example), the stream will deposit excess sediment in the channel as riffles, bars, or islands. A dramatic reduction in sediment, such as construction of a dam that traps sediment will cause the downstream channel to enlarge by widening and down cutting.
The relation between discharge and sediment transport is important because the complex interactions between these elements determines whether the stream channel is stable, aggrading or degrading (Lane 1955).
Bankfull flow is generally defined as the height of the floodplain surface or the flow that "just fills the stream to its banks" (Gordon et al. 1992; U.S. Forest Service 2003) or the stage at which water starts to flow over the floodplain (Dunne and Leopold 1978). Bankfull flow is subject to minimum flow resistance (Petts and Foster 1985) and produces the most sediment transport over time (Inglis 1949; Richards 1982). Bankfull events have a recurrence interval of approximately 1.1-2.0 years. Scour of fines from pools and deposition of bedload in riffles is also most predominant at bankfull flow, creating and maintaining high quality habitat. (Leopold et al. 1964; Mosley 1981; Rosgen 1996).
High water is a regular occurrence in unimpounded rivers, such as the Yellow Medicine River in south central Minnesota. Over bank flow serves to rejuvenate the floodplain and riparian vegetation, form channels, scour in-channel habitat, and attenuate the severity of flooding downstream. Such resetting of the system is an essential process.
The free play of a stream’s hydrodynamic forces creates a rich diversity of habitats for aquatic life. This diversity results from variation in stream depth, width, water velocity, and substrate throughout the channel, (the antithesis of a channelized ditch). Riffle habitat provides feeding locations for many species of fish and spawning habitat for species such as walleye, lake sturgeon, trout, darters, and suckers. Many aquatic invertebrates rely on riffle habitat. Pools and eddies provide spawning and feeding areas for species such as smallmouth bass, and provide some invertebrate species with slow water and finer sediment.
It is important to recognize that the physical habitat essential to maintaining the aquatic community is formed by periodic disturbance. This disturbance in the short-term may be detrimental to the growth and survival of individual fish or entire year classes of fish. On the other hand, high flows reset the system. Formation of new channels, scouring vegetation and fine sediments, abandoning side channels; all work to create habitat beneficial for some species over the long-term. Periodic resetting of the system is an essential process.
The interactions between the variables that create a stream channel are complex and multi-dimensional. Continued study of these interactions is essential to understand the consequences that intended and unintended alterations can have on the health of the entire watershed system.