Instream Flows for Recreation: A Handbook on Concepts and Research Methods

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LANDSCAPE POSITION AND RIVER CLASSIFICATION Rivers are integral parts of the landscapes through which they flow. When describing river systems it is important to understand and identify the interdependency between rivers and their valleys. For example, is the river actively downcutting? Does the river flow through alluvial sediments where the channel is seasonally being reworked by sedimentation processes associated with various flow regimes? Is the river relatively unconstrained (e.g., the floodplain is several times greater than the bankfull channel) or is the river channel largely constrained by bedrock or human -built structures? Is the stream flowing through old valley - bottom lake deposits or other formations created by pre - historic geologic processes? The answers to these or related questions are at the heart of many long- term or indirect changes that may occur when a streamflow regime is changed. Accurate descriptions of the landscape setting and geologic conditions are useful for understanding river conditions on almost any type of river, but this information is particularly important for examining geomorphic features that result from ongoing sedimentation processes. Depositional features such as riffle substrate, bars, beaches, and floodplains may represent critical components of overall stream character and they are often highly sensitive to changes in an instream flow regime. Depositional features are, by definition, pervasive and important on alluvial streams. In addition, they can also be prominent in bedrock or boulder streams, forming critical fish or wildlife habitats in backwaters or eddies and providing features important for recreation. For example, while the Colorado River in the Grand Canyon is a downcutting, geologically - controlled river, bars and beaches created by eddy flow in backwaters are essential components of the riparian ecosystem and provide high quality camping areas for recreationists. There are several methods of river classification, all of which convey considerable information on river conditions and associated fluvial processes. For example, at the most basic level rivers can be characterized as either bedrock or alluvial, providing a starting point for information about the general character of a stream over a range of flows. However, rivers can be further classified based on the principal mode of sediment transport, landscape position, or other variables. For a brief discussion of these classification systems, see the sidebar on page 23. unlikely to have large effects here, although other reaches feature flow - sensitive bars and beaches. 22