UNIVERSITY OF NEW MEXICO

A common assumption underlying ecological and hydrological views of small stream systems is that beaver damming causes long-term filling of valleys with sediment, creating broad, flat ?beaver meadows?. Thus beaver activity is thought to promote both a persistent reduction in sediment flux and greater groundwater storage capacity in the accumulated deposits. By reducing flow velocities, individual beaver ponds clearly trap sediment, but the depth and spatial distribution of beaver-related sediment deposition have never been quantified over timescales longer than the lifetime of a single beaver pond. The goal of this project is to document beaver-related sediments in floodplains of small streams in the greater Yellowstone area, in deposits dating from the present back to the end of the last glaciation about 12,000 years ago (the Holocene epoch). By detailing the texture, mineral and organic composition, chemistry, and sedimentary structures of modern beaver-pond deposits and their associated landforms, this work will define diagnostic characteristics essential to identifying beaver-related deposits in older floodplain sequences and abandoned channels. Mapping and thickness measurements will allow quantification of the abundance of beaver-pond deposits relative to undammed stream deposits, and the total contribution of beaver-related sedimentation to net filling of valleys. Beaver cannot dam streams that are too large, steep, or confined within narrow valleys. Therefore, streamflow, channel and valley slopes, valley width, meander geometry, and other geomorphic factors will be measured for both modern and Holocene beaver dam sites to evaluate which segments of the study stream network are conducive to beaver damming and consequent sedimentation. Radiocarbon dating will allow the timing of beaver-pond deposits to be understood in the context of long-term environmental change in the Yellowstone region. Preliminary results in northern Yellowstone imply that channel slope and drainage basin area limit suitable dam sites for beavers, and that the magnitude of net valley filling from damming is small along most stream reaches. The temporal distribution of deposits also suggests that beaver became scarce, or limited to larger streams, during prolonged severe droughts. Expansion of the study to a greater range of stream environments in greater Yellowstone area will allow the hypothesis of regional climatic control on beaver effects and overall stream behavior to be tested. Knowledge gained in this study about the long-term effects of beaver damming is of significance in understanding the long-term evolution of stream valleys. It also has relevance to improving the overall health of small stream systems, e.g., in assessing where channel incision and other impacts stemming from the loss of beaver have occurred, and in designing stream restoration projects where necessary to counteract these impacts. It will thus help to focus available resources for restoring and protecting stream habitat and water quality on those waterways that are most in need. Such information is particularly relevant to management in northern Yellowstone National Park, where a lack of beaver damming is thought to have caused downcutting of many streams, with consequent lowering of water tables and reduction of wetlands habitat; yet, initial results of this study suggest that these problems are not as widespread or severe as previously concluded. The study will involve independent research projects and advanced scientific training for two graduate students. In addition, undergraduate students from the predominantly Native American population at the UNM-Gallup campus will be involved in both field and laboratory research that will assist in movement of these students into scientific degree programs and careers.

Clarification of Codes