REINTRODUCED BEAVERS RAPIDLY INFLUENCE SEDIMENT STORAGE AND BIOGEOCHEMISTRY IN HEADWATER STREAMS OF THE METHOW RIVER, WASHINGTON, USA

North American beavers were nearly decimated from headwater landscapes by European loggers and fur traders in the 18th century, as beaver populations dropped from an estimated 40-600 million individuals to 6-12 million individuals today. The legacy of beaver extirpation, and the diverse role of beavers in fluvial ecosystems are well understood. The loss of beavers from surface waters has had profound effects on riparian zones, including channel geomorphology, temperature regimes, sediment storage, channel-floodplain connectivity, and carbon and nutrient dynamics.

Our study presents a unique opportunity to characterize the short-term temporal dynamics of how beavers influence sediment organic material accumulation in 1^st and 2^nd order streams. We selected 5 sites with beavers reintroduced over the past 5 years by The Methow Beaver Project. We aim to understand how rapidly beaver bioengineering influences sediment organic material accumulation within stream segments that would otherwise store little to no organic-rich sediments. Sources of beaver related organics include coarse woody debris, fecal matter, and allochthonous material. At each site we mapped the extent of beaver impacted areas, collected shallow sediment cores from behind beaver impoundments, and upstream and downstream of each dam site. We then compared those samples to sediments in adjacent wetland areas not created by beaver activity.

Beaver wetlands formed in just the past 5 years are up to 2800 meters² in area, and contain sediments that are >1 meter deep. We measured sediment physical properties, and analyzed samples for weight percent carbon and nitrogen (%C and %N). Our preliminary results show substantial differences in sediment accumulation, %C, and C/N ratios in beaver versus non-beaver samples. %C and C/N ratios in sediments from beaver impoundments reached 12% and 36. In contrast, non-beaver sample %C and C/N ratios were as low as <1% and 10. These findings suggest that beaver wetlands store substantial volumes of organic-rich sediments that are largely sourced from terrestrial biomass. Our temporally constrained results provide insight into the rapidity at which beavers can influence biogeochemical systems (including carbon and nutrient storage) in headwater stream sediments.