Historical Riparian Habitat Changes of an Endangered Bird Species: Interior Least Terns along the Red River below Denison Dam
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The Red River along the Texas-Oklahoma border provides ideal Interior Least Tern (Sterna antillarium athalassos) habitat because of its wide river channel with large, open sandbars that are sparsely vegetated and close to aquatic food sources. Over the last century, tern sandbar nesting habitat along the Red River has been lost or gained in response to water resource projects, dams, reservoirs, floods, droughts, land cover changes, and invasive vegetation. This projects presents the spatial and temporal changes in tern sandbar habitat on a 170-km reach of the Red River below Denison Dam that have occurred since the 1890s. Specifically, this project investigates how the hydrology, land cover, sediment budget, channel geometry, sandbar area, and suitable tern habitat area has changed from the late 1890s to 2014. Historical surveys and aerial photography were used to map floodplain land cover and calculate channel geometry and sandbar area. Suitable tern sandbar habitat was delineated based on three main criteria: minimum distance of 76-m from tall trees or shrubs, no vegetation on sandbar, and minimum distance of 60-m from channel margins. Streamflow and precipitation data from this time period were used to assess hydrology changes, and sediment yield estimates were gathered from previous studies and compared to suspend-sediment gage information within the study area. Sandbar area was variable over the 118-year timeline, tending to increase when channel width and specific stream power increased. Suitable tern sandbar habitat overall decreased by about 0.7%, largely due to decrease in channel width and increase in vegetated sandbar area. Lack of high flows after the completion of Denison Dam decreased specific stream power, allowing vegetation to encroach on sandbars, decreasing both channel width and suitable habitat area. These interrelationships between hydrology, land cover, sediment, channel geometry, sandbar area, and suitable tern sandbar habitat area will help inform future management policies for the Red River, as well as other threatened riparian bird species.