THE IMPACT OF BASIN GEOMORPHOLOGY ON THE SEDIMENT SIZE OF FINE-GRAINED BEAVER-RELATED DEPOSITS IN NORTHERN YELLOWSTONE NATIONAL PARK

In northern Yellowstone National Park beaver damming increases riparian habitat along many streams. The beaver’s ability to create a dam that causes sediment deposition depends on the type and size of building material used by the beaver. Aspen or willow trees, mud, and rocks are used in the construction of beaver dams resulting in low velocity zones immediately upstream of the dam, creating pond areas. Beaver-related deposits are recognized in terrace stratigraphy be the presence of abundant organic matter and beaver-chewed twigs. A range of sediment thickness and particle sizes are associated with late Holocene beaver-related deposits in northern Yellowstone National Park. The grain size of sediments associated with beaver damming is potentially a function of the size of the drainage basin because of higher discharges associated with larger basins that may limit beavers’ ability to build dams that sufficiently reduce water velocity for deposition of clay. Therefore beaver-related deposits on larger streams may have less clay and larger median grain sizes. Alternatively, beaver-related sediment on larger streams may be deposited in other low energy environments such as beaver-forced overbank floods where more clay is deposited.

Initial observations show that late Holocene beaver-related deposits are mixtures of fine-grained sediment and organic debris that is best characterized by soil texture nomenclature (relative to abundances of sand, silt and clay). Larger drainage basins such as the Gardner River (basin area = 95.0 km²) contain beaver-related deposits comprised of sandy clay loams with clay and sand abundances ranging between 20-35% and 45-80%, respectively. Smaller drainage basins, such as Geode Creek (basin area = 0.66 km²), contain more silty-sand loams with clay and sand abundances ranging between 5-20% and 30-60%, respectively. The smaller percentage of clay in beaver-related deposits on the larger streams may be the result of differences in the types of depositional environment associated with beaver on larger streams. On larger streams, floods may cause more frequent dam failures resulting in overbank floods that deposit more clay on floodplains. Future analysis of particle size by laser diffraction will provide more precise estimates of clay abundance in these deposits.