ASSESSMENT OF LAND-USE IMPACTS ON FORCED-POOL CHARACTERISTICS IN CONSTRICTION-DOMINATED CHANNELS

Historical and contemporary land use practices that have occurred in Colorado for the last two centuries have altered the supply of sediment and water to many channels in the upper South Platte Basin. As a result of increased sediment supply and mobility and reduced erosivity of high flows, the characteristics of pools associated with channel constrictions, referred to as forced pools, have been altered and important habitat diversity has been lost. It was expected that pool characteristics in impacted reaches would be statistically different from controlled reaches, and alterations in the sediment regime would show greater differences than changes in the flow regime. In order to assess these alterations, pool volume (length x bank-full width x residual depth), downstream spacing and constriction characteristics (geometry and rock-mass strength) of 20 consecutive forced pools were surveyed on five rivers in the upper South Platte Basin. Four of the five rivers were associated with different land use practices, which included contemporary road corridors, contemporary flow regulation, historic placer mining, and historic tie drives, while one river served as a control with no associated impact. Study reaches were selected based on average gradient (0.01) and an abundance of laterally constricted pools. The difference in pool volume between impacted and controlled reaches was compared. In addition, pool spacing was compared to an existing theoretical model developed for constriction-dominated channels. Preliminary results indicate that controlled reaches have greater pool volume and downstream pool spacing than impacted reaches. The final results from this study will specifically serve as a framework for restoration and management of constriction-dominated rivers in the upper South Platte drainage basin and as a guide for other constriction-dominated western rivers that were subject to similar human disturbances.