Channel Changes on the Middle Rio Grande

Abstract

The Middle Rio Grande (MRG) has changed dramatically over the last century as a result of both natural and human-caused alterations in the watershed and channel. The observed channel morphology is constantly changing as the MRG seeks to balance the movement of sediment (sediment supply) with the energy available from the flow of water (sediment transport capacity) (Schumm et al., 1984). An imbalance between sediment transport capacity and sediment supply is a key cause of most channel and flood plain adjustments (Lane, 1955; Schumm, 1977) and this condition has been observed on the Middle Rio Grande. Factors affecting the imbalance between sediment transport capacity and sediment supply have been categorized as drivers of adjustment and controls on adjustment. Important drivers on the MRG include flow frequency, magnitude and duration; and sediment supply. Controls on recent channel adjustments on the MRG include bank stability, bed stability, base level, flood plain lateral confinement, and flood plain connectivity. The influence of drivers and controls along the MRG is variable, but commonalities have been identified. It is the commonalities in the river’s responses to drivers and controls present that help identify and separate the MRG into reaches with similar trends. The relationship between sediment transport capacity and sediment supply is fundamental to anticipating future changes in reach trends and the direction of river responses. Observed reach-based trends include Channel Narrowing, Vegetation Encroachment, Degradation, Increased Bank Height, Bank Erosion, Coarsening of Bed Material, Aggradation, Channel Plugging with Sediment, and Perched Channel Conditions.