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 usin g data from 1875 to the present shows that along four reaches relative ly abrupt shifts of the channel to a new location (avulsions) have bee n common, but in five other reaches the channel has been laterally sta ble. In the unstable reaches, repeated avulsions have led to the devel opment of subequally spaced, lozenge-shaped internodes composed of coa rse-grained point-bar deposits and both coarse-and fine-grained abando ned-channel fill. In the stable nodes that separate the active interno des deposition has been minimal. In map view, the large-scale depositi onal geometry of these nodes and internodes is analogous to a string o f lozenge-shaped, linked sausages, the links representing the nodes an d the lozenges representing the internodes, Two end-member geometries could result from the aggradation of the nodes and internodes: (1) lat erally swelling and thinning, mostly coarse-grained, channel-derived d eposits (internodes) enclosed by fine-grained overbank sediments in sy stems dominated by overbank sedimentation, and (2) vertically and late rally amalgamated nodes and internodes composed primarily of coarse-gr ained deposits in systems dominated by channel-derived sedimentation. Rio Grande mean annual discharge decreased by 60%-70% from 1875 to 192 5 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 f rom other studies suggest that this was probably a result of decreased discharge. (2) Sinuosity increased from about 1.2 to 1.7, and the num ber of two-channel reaches appears to have decreased, probably as a di rect result of the avulsion process. During an avulsion, the new chann el evolves from low to high sinuosity due to rapid meander growth at t he same time that discharge shifts from the old to the new channel. As a result, early in an avulsion two channels of different sinuosity ge nerally are present, but later a single high-sinuosity channel develop s. Thus, the observed decrease in avulsion frequency may be responsibl e for the present high number of single channel, high sinuosity reache s. (3) The number of avulsions that occurred decreased from about 19 ( 1875-1941) to 2 (1941-present). Data are inadequate to show what, if a ny, relationship exists between the decrease in discharge and the chan ge in avulsion frequency.