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<br />sinuosity, and sediment size depends on the relative increases of those variables. For <br />example, if the increase in discharge is significantly greater than that of sediment load, <br />sinuosity will increase, sediment size will increase, and depth will increase. Conversely, <br />if the increase in sediment load is larger than that of discharge, sinuosity will decrease, <br />sediment size will decrease, and depth will decrease. Impacts of excessive grazing <br />commonly includes channel adjustments such as accelerated bank erosion; increased <br />width/depth ratios; altered channel patterns; induced channel instability; increased <br />sediment supply; and decreased sediment transport capacity (Rosgen, 1966). <br /> <br />6.2. Influence of Flow Augmentations on Channel Dynamics <br /> <br />Although research has been conducted regarding the effects of flow diversions and <br />consequent flow reductions on channel dynamics (Ryan, 1997), relatively little work has <br />been conducted regarding the geomorphic impacts of flow augmentations. In fact, the <br />Upper Arkansas River basin has provided a primary research location for such <br />investigations (Abbott, 1976; O'Neill, et. ai, 1997, Dominick and O'Neill, 1998). The <br />fundamental impact of flow augmentations is an increase in stream power, which is the <br />product of discharge and water surface slope. Additional stream power increases the <br />ability of a channel to move coarser sediment fractions, which results in channel erosion, <br />and coarsening of the mobilized and remaining bed material. Eventual channel stability <br />is reached when the channel size is adjusted to accommodate the new flow condition, and <br />the sediment transport rates are adjusted accordingly. The rate of this recovery is <br />dependent on the adjustability of the system relative to the magnitude of the change. <br />System adjustability is dependent on the erodibility of the channel perimeter, the quantity <br />and caliber of the upstream sediment supply with respect to the transport energy, and the <br />flow energy available to drive adjustment. <br /> <br />Abbott (1976) was first to observe and measure changes in channel morphology caused <br />by transbasin imports in the Lake Fork basin above Turquoise reservoir. He found the <br />channel adjusted to the new flow regime by widening, degrading its bed, and <br />straightening. Dominick and O'Neill (1998) found that augmented Arkansas River basin <br />channels accommodate the increase in stream energy by widening and coarsening. These <br />channel adjustments increase sediment loads downstream, causing channel braiding and <br />widening. <br /> <br />Another driving force for geomorphic adjustment is rapid drawdown, which commonly <br />results from flow release practices at reservoirs. Rapid drops in discharge cause a <br />commensurate drop in channel stage, which is generally referred to as rapid drawdown. <br />Rapid drawdown commonly results in a stage (water surface elevation) drop that is faster <br />than the drainage rate of the channel banks, which can result in gravitational failure of the <br />saturated bank strata. The process can result in high rates of channel widening, Rapid <br />discharge fluctuations also have the potential to affect sediment transport conditions <br />within the channel. Sediment within riffles and bars that is mobilized during high flow <br />events is progressively deposited on the falling limb of the hydrograph, promoting the <br />redevelopment of bars, riffles, and pools, which are all indicators of effective sediment <br />transport and geomorphic stability. If the drop in flows is extremely rapid, however, <br />there is little opportunity for the channel to reform those features due to the lack of <br /> <br />May 7. 1999 <br /> <br />Fluvial Geomorphological Assessment <br />Upper Arkansas River <br /> <br />Page 36 <br />