The Influence of Floodplain Large Wood Jams on Soil Moisture in the Colorado Front Range, USA

Large wood (LW) in river corridors (channels and floodplains) influences geomorphic processes and provides nutrients, organic matter, sites of vegetation establishment, and habitat for biota. However, there have been limited studies on the effects of LW in floodplains. Previous work has indicated that enhanced soil nutrients and seedling establishment associated with floodplain LW accumulations (i.e., jams) could be influenced by increased soil moisture underneath floodplain LW jams. We tested whether LW jams influence underlying soil moisture in floodplains by taking soil moisture samples of areas underneath jams and control areas without jams to test for significant differences. We collected soil moisture measurements in floodplain soil on West Creek and Roaring River, two rivers in the Rocky Mountains, Colorado. These rivers have significant amounts of LW deposited in the floodplain from major flooding events in 2013 (West Creek) and 1982 (Roaring River). We found large variability in the soil moisture measurements that we collected, highlighting the need for continuous monitoring of soil moisture throughout the year. Contrary to our initial expectations, we found no significant differences in soil moisture underneath jams compared to measurements taken adjacent to jams. In measurements taken just after recent rainfall, we found higher soil moisture adjacent to jams compared to underneath jams, due to the jams protecting the subsurface from precipitation. Soil color (indicating organic matter content) and grain size strongly influence soil moisture, with lighter and coarser soils containing lower soil moisture. Surficial organic layer thickness is higher at locations underneath jams compared to adjacent to jams, indicating that as organic material decays and becomes incorporated into the subsurface, there may be an influence on soil moisture underneath jams in the future. A better understanding of physical and ecological effects caused by LW in floodplains will inform management of LW in river corridors.