Quantifying long term spatial dynamics of river channel change. South Platte River, Logan County, Colorado

Keywords: Riparian, floodplain, GIS
Session Type: Virtual Paper
Day: Thursday
Session Start / End Time: 4/8/2021 04:40 PM (Pacific Time (US & Canada)) - 4/8/2021 05:55 PM (Pacific Time (US & Canada))
Room: Virtual 3

Abstract

In eastern Colorado, the South Platte River supports a broad cottonwood gallery forest that both provides important ecosystem services and consumes ample quantities of water, a much sought-after resource in this semi-arid environment. The forest became established between the 1880s and 1930s, as a result of water management, including agricultural water use and return flows, dams, diversions and trans-basin water imports. During the process of forest expansion, the channel narrowed from a dynamic braided system to a more stable system with fewer channel threads. We developed methods to quantify the spatial patterns and dynamics of the anthropogenically-narrowed active river channel for a 30-km river section in Logan County, Colorado. We digitized the active channel (defined as the area open water and <50% vegetated sand bars) in eight sets of historic aerial images, collected at roughly decadal intervals between 1941 and 2016. We used spatial analysis to examine the changes in overall channel area, and spatial patterns of channel location over time. We found that the active channel expands and contracts at a decadal scale, in association with streamflow conditions. For example, during a period of high flows (2006 – 2015), we observed a 40% increase in overall channel area and during a drought period (1998-2006) we recorded a 16% decrease in channel area. These dynamics appear to coincide with expansion and contraction of forest cover and are likely sufficient to enable cottonwood-willow forest regeneration, albeit in a much smaller area than formerly occurred.