Effects of River Channel Expansion and Contraction on Riparian Vegetation, South Platte River, Logan County, Colorado, 1956-2016.

Authors: Haley Hampstead*, , Dr. Gabrielle Katz, Department of Earth and Atmospheric Sciences, Metropolitan State University of Denver, Dr. Jessica Salo, Department of Geography, GIS and Sustainability, University of Northern Colorado
Topics: Biogeography
Keywords: Biogeography, land cover, riparian, GIS
Session Type: Poster
Day: 4/5/2019
Start / End Time: 3:05 PM / 4:45 PM
Room: Lincoln 2, Marriott, Exhibition Level
Presentation File: Download

Over the past 150 years, cottonwood forests have spread along the South Platte River in northeastern Colorado. This expansion was associated with water management beginning in the 1880’s, which stabilized and augmented stream flows. As a pioneer species, intermittent flooding is required for the establishment of cottonwood seedlings. However, large floods can also remove trees, creating complex patterns of hydrology-forest interactions over time. This research documents historic spatial patterns of cottonwood forest development to aid future resource management. We posed the question, “How did historic river channel change affect the riparian forest?” To answer this, we digitized riparian vegetation using orthophotos at roughly decadal increments for a 30 km section of the South Platte River in Logan County, Colorado. We then assessed change in the majority land-cover type in each 60 m² cell over time, allowing us to measure the rate of change of forest, riparian herbaceous vegetation, and the active channel within each time interval. We found that years of low stream-flow were associated with high expansion rates for herbaceous vegetation and forests. From 1998-2006 when the channel loss was 9.6 hectares/year, the forest expanded by 29.8 hectares/year, while herbaceous vegetation expanded by 28.2 hectares/year. In contrast, high flow periods were associated with high loss rates for herbaceous and forest cover. From 2006-2015 when channel expansion was 15.9 hectares/year, forest loss occurred at 20.2 hectares/year, while the rate of herbaceous loss was 46.0 hectares/year. These results indicate that riparian vegetation cover is sensitive to stream flow at decadal intervals.

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