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. . . .' ., ~ lZ:..-" .... / TROll'l' UNLlMITID Colorado Water Conservation Board 1313 Sherman Street Room 721 Denver. CO 80203 May 11,2007 RE: Effects of inundation on instream flow values Dear Directors, The rules governing Colorado's Instream Flow Program call for Staff to evaluate how inundation of a stream reach impacts existing ISF rights. As part of this evaluation, Staff must consider whether or not", .. the ISF right or natural environment will be adversely affected by the inundation." Understanding how impoundments differ from free-flowing streams is integral to making this evaluation. This letter is intended as a primer on how inundation affects free-flowing streams. Water flows downhill, Virtually every physical and biologic component of a stream channel is heavily influenced by this downhill movement of water. The shape of a stream channel is largely a product of the interactions between flowing water and sediment supply. As water moves down hill, it picks up sediment and carries it downstream, carving out the stream channel. Because the maximum size of stones and boulders that can be moved by a stream is highly dependent on the amount of water moving downstream. channel fonn is often determined by peak flows associated with large floods. These erosional processes are not only responsible for cutting the scenic oxbows and canyons that characterize many streams, but also for keeping the larger stones and cobbles that make for good spawning habitat from being buried under fine sediments. In addition to moving sediment downstream. the directional movement of water has other important effects on streams. Flowing water carries nutrients and pollutants downstream, limiting their accumulation in upstream habitats. Because streams tend to be turbulent with water cascading over rocks and other debris, streams are typically well oxygenated. The downhill flow of water even affects the shape and behavior of organisms living in the stream. The streamlined bodies and rounded cross-sections of many fish species, including trout. not only enable them to swim fast but may help them swim in high velocity currents. Conversely, laterally flattened fishes, like sunfish, while thought to be less well suited to speed are viewed as more maneuverable and better adapted for slower-moving water. Other researchers have shown that animals choose habitats that allow them to take advantage of the food flowing water delivers from upstream. For example, caddisflies are a group of insects that spin nets which they use to capture food from the water column, Where they chose to place these nets is influenced by current velocity, with some species preferring higher velocities and others preferring slower- moving water. . Dams dramatically alter movement of water and its influence on aquatic organisms, As the reach above a dam is inundated, surface water tends to spread out and slow down, The result is a relatively slow moving body of water whose surface area and depth are much greater than the stream it replaced, Because horizontal water movementethrough this reservoir is no longer rapid, sediments drop out of the water column and accumulate behind the dam. These sediments can bury important stream habitat. including spawning beds. Additionally. because water spilling over the dam is comparatively sediment-starved it ten~ to pick up sediment as it moves downstream. causing Trant U,dimited: America's Leadi", Coldwater Fisl,eries C01,seroatio" Orga"izatio" 1320 Pearl Street, Suite 320, Boulder, CO 80302 (303) 440-29370 · Fax; (303) 440-7933 · www.tu.org