CHANNEL AND ISLAND EVOLUTION IN THE EASTERN PLATTE RIVER, NEBRASKA, 1938-1999

Channels and islands in eastern Platte River (EPR) from Grand Island to Louisville, Nebraska changed markedly during the period 1938-1999. In four selected EPR stretches of 20.0-27.7 km in length, total river-channel surface area (TCSA) decreased between 19% and 45% from 1938/1941 to the end of the 1960s. Decreasing river widths, however, show different temporal patterns at different points within individual stretches, reflecting complex changes in stream planform, confluence mechanics, bar accretion, and anabranch development/abandonment. The greatest decreases in TCSA occurred in two stretches upstream from the confluence of the Platte and Loup rivers at Columbus (CC, SC), Nebraska. Simultaneously, there were large increases (245% and 350% of baseline1938/1941 values) in CSIA in the same two stretches (CC, SC) prior to 1968-1971. In comparison, there were only moderate to slight (75%-95% of baseline 1938/1941 values) decreases in CSIA in a stretch immediately downstream from the Platte-Loup confluence (RG) and in another immediately downstream of the mouth of the Elkhorn River. After 1968-1971, CSIA decreased sharply in stretches CC and SC, but in stretches RG and EP, CSIA decreases continued at a lesser rate in continuity with a trend that began in the mid-1950s. Changes in the distribution patterns of stabilized islands of different size classes are also apparent during 1938-1999. Stretch CC, the farthest west of all of the stretches, shows the least change over time in the distribution pattern of islands by area, whereas stretch EP shows the most change. Temporal patterns of CSIA after 1970 overlap statistically significant changes in discharge during the study period, and may correspond in large measure to increases in discharge during the month of August. These changes in discharge have been driven, in part, by evolving irrigation policies and procedures.

Our robust, GIS-based analysis of historical change on the EPR expands upon work carried out upstream on the Platte and North Platte Rivers in the 1970s and 1980s. In general, our results agree with the results of earlier studies, but it is clear that new insights emerge from the quantitative analysis of entire stretches, as opposed to river-width measurements or qualitative assessments of in-channel change alone.