8188 - Laserfiche WebLink

Home Browse Search

Filial Report 3-40 September 2000 3.6.2 Sediment Dynamics, Shoreline Complexity, and Low- Velocity Habitats Within a particular reach, shoreline complexi ty is affected by sediment-deposition processes and geologic conditions. Consequently, shoreline complexity varies considerably among different planform types. An understanding of shoreline complexity is important because it affects the distribution and suitability of habitats, including backwaters and other low-velocity habitats used as nursery areas by the endangered fishes, especially Colorado pikeminnow and humpback chub. Direct measurements of shoreline complexity calculated from topographic maps of compound bars in different study reaches show that complexity is greatest at those discharges when the bar surface is partly inundated and where chute channels are inactive. At a very low river stage, complexity is determined by the topography of the bar margins, which are typically simpler in shape than are the upper-bar surfaces. When higher discharges inundate the bar surface, complexity is determined by the planform of the floodplain edge. Olsson and Schmidt (1993) showed that the elevation of greatest shoreline complexity changes from year to year because the elevation and topographic complexity of bars change depending on the hydrologic regime during spring runoff. The longitudinal distribution of channel planforms (Section 3.6.1) for the Green River affects the longitudinal distribution of shoreline complexity, and the locations of high or low complexity change with discharge. Restricted meanders have considerable shoreline complexity at bankfull discharge because of the presence of vegetated mid-channel islands. In contrast, fixed meanders have relatively little available habitat at bankfull discharge because the banks are relatively smooth and there are few permanent mid-channel islands. At intermediate stages, complexity increases dramatically, and some segments have significantly more complexity than other segments. At a very low stage, there is little difference in habitat complexity between fixed and restricted meanders, but these segments have higher habitat complexity than canyons (Schmidt 1996). Except at very low flow, shoreline-complexity indices can be relatively high in canyons with abundant debris fans. In contrast to alluvial reaches, whose banks typically have smooth transitions from one orientation to another, debris-fan segments have banks that are composed of coarse, angular deposits where bank orientations have sharp angles. These divergences give rise to low-velocity habitats even at high river stage. Shoreline complexity - A geomorphic variable that describes the degree that shorelines deviate from a straight line, calculated by dividing the totallength of both shorelines by the mid-line of the channel. Low-velocity habitat - An area within the river channel that has lower flow velocity than the main channel of the river. These areas provide refuge for fish and allow them to conserve energy and are particularly important for larval and young- of-the-year fish. Portions of the river with complex shorelines generally have more low-velocity habitat. Backwater - A generally shallow area within the river channel with little or no flow that is situated downstream of an obstruction, such as a sand or gravel bar, and that has some direct surface water connection with the river. Eddy - An area downstream of an obstruction within the river channel where the local current moves against the main current in a circular motion.