Assessing the Stability and Age of Riverine Islands of the Payette River Using Soil Characteristics, Historic Aerial Photos, and Remote Sensing Techniques

Mid-channel islands in rivers, or riverine islands, fulfill numerous different roles from providing habitat heterogeneity to hosting permanent human settlements. Analysis of soils, historic photos, and remote sensing techniques provide insights into the age and stability of riverine islands. Dams placed upstream of riverine islands in the North Fork of the Payette River 1940’s -1950’s starved islands of sediment which could lead to a reduction in island area; however, decreased peak stream flows due to upstream dams could increase island stability. Soil textures on the riverine islands consistently were > 90% sand composition, and nearby floodplains were either loamy sand or sandy-clay loam texture. Soils on both islands and floodplains are fluvent soils, and the riverine islands showed little in terms of soil horizon development. There is little difference in relief between the islands (gradient < 2.0%). Ponderosa pine trees on the riverine islands, willows, and other undergrowth vegetation indicate island stability, likely related to discharge control provided by the dams. Since the dams were constructed, peak annual discharge on the North Fork of the Payette River has never exceeded 2200 cfs. Historical, aerial photographs dating back to the 1990’s reveal an increase in island size and vegetation.