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

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FLUVIAL AND GEOMORPHIC PROCESSES An important aspect of many instream flow studies is to interpret the effects of alternative flow regimes on river form and process. This is a difficult issue for which there are generally only descriptive rather than quantitative tools. Where available, historical aerial photographs over several time periods can provide an excellent perspective for understanding the types of channel changes possible and expected. In some instances, physical modeling may offer a way to explore potential channel changes under different flow regimes at specific sites. However, these or similar analyses will not always provide enough information to accurately predict the way a river will change in response to various flows or sediment inputs. Nevertheless, accumulated knowledge about these relationships will often allow scientists to forecast the direction and perhaps the general magnitude of channel responses to flow or sediment changes. Rivers flowing through fluvial - deposited sediments are the most susceptible to flow regime changes — especially changes in high flows. In contrast, bedrock channels generally experience little change in channel morphology regardless of flow regime. When conducting an analysis of channel morphology response to different flows, it is useful to employ several different tools. First, the effect of alternative flows on sediment balances needs to be assessed. Next, based upon channel classification and river morphology responses to changes in flow or sediment, an assessment should be made as to overall morphologic response to changed flow and sediment regimes (see sidebars on pages 27 -29). Finally, a deductive assessment of how individual morphologic features, such as bars, respond to altered flow and sediment regimes may be required. Because of the complexity of flow and sediment transport interactions, an assessment of sediment balance response typically needs to be made on a case -by -case basis. Large dams both reduce sediment transport capacity and eliminate sediment delivery from upstream areas. This usually results in an excess of transport capacity immediately downstream from dams, downcutting channels, and an increased rate of erosion of depositional features. However, further downstream, if tributary sediment inputs are great, the now - reduced transport capacity of the mainstem may be overwhelmed by tributary sediment which might in turn accelerate sediment aggradation (the filling -in of the channel). Similarly, stream diversions tend to reduce sediment transport capacity in relation to sediment load, increasing sediment storage in depositional zones along a particular reach. One useful conceptual tool for understanding these relationships is the sediment /water scale shown in Figure 17. Any change in one parameter changes the others as well. rigure ii. ine seaiment - water scale snowing reianonsnips between sediment inputs, flow inputs, and channel change. The balance between flow and sediment inputs determines how a channel will evolve. 24