Channel avulsions and related processes, and large-scale sedimentation patterns since 1875, Rio Grande, San Luis Valley, Colorado

Analysis of a section of the Rio Grande in south-central Colorado using data from 1875 to the present shows that along four reaches relatively abrupt shifts of the channel to a new location (avulsions) have been common, but in five other reaches the channel has been laterally stable. In the unstable reaches, repeated avulsions have led to the development of subequally spaced, lozenge-shaped internodes composed of coarse-grained point-bar deposits and both coarse- and fine-grained abandoned-channel fill. In the stable nodes that separate the active internodes deposition has been minimal. In map view, the large-scale depositional geometry of these nodes and internodes is analogous to a string of lozenge-shaped, linked sausages, the links representing the nodes and the lozenges representing the internodes. Two end-member geometries could result from the aggradation of the nodes and internodes: (1) laterally swelling and thinning, mostly coarse-grained, channel-derived deposits (internodes) enclosed by fine-grained overbank sediments in systems dominated by overbank sedimentation, and (2) vertically and laterally amalgamated nodes and internodes composed primarily of coarse-grained deposits in systems dominated by channel-derived sedimentation. Rio Grande mean annual discharge decreased by 60%-70% from 1875 to 1925 because of irrigation withdrawals upstream from the study area. From 1875 to the present the following changes have occurred: (1) Meander wavelength decreased from about 500 m to about 320 m. Empirical data from other studies suggest that this was probably a result of decreased discharge. (2) Sinuosity increased from about 1.2 to 1.7, and the number of two-channel reaches appears to have decreased, probably as a direct result of the avulsion process. During an avulsion, the new channel evolves from low to high sinuosity due to rapid meander growth at the same time that discharge shifts from the old to the new channel. As a result, early in an avulsion two channels of different sinuosity generally are present, but later a single high-sinuosity channel develops. Thus, the observed decrease in avulsion frequency may be responsible for the present high number of single channel, high sinuosity reaches. (3) The number of avulsions that occurred decreased from about 19 (1875-1941) to 2 (1941-present). Data are inadequate to show what, if any, relationship exists between the decrease in discharge and the change in avulsion frequency.