CS08 Biogeochemical Cycles
Date: Monday, June 10, 2002
Time: 2:45:00 PM
Location: Saanich
 
Triska, F, J, USGS, Water Resources Division, Menlo Park, CA, USA, fjtriska@usgs.gov
Jackman, A, P, University of California at Davis, Davis, CA, USA, apjackman@ucdavis.edu
Duff, J, H, USGS, Water Resources Division, Menlo Park, CA, USA, jhduff@usgs.gov
Avanzino, R, J, USGS, Water Resources Division, Menlo Park, CA, USA, avanzino@usgs.gov
 
BEAVER DAMS RETAIN DIN AND POC FOR IN SITU PROCESSING: EVIDENCE FROM A NEWLY CONSTRUCTED DAM IN A SAND-BED STREAM
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When beavers colonize lowland streams they profoundly impact the channel by flooding riparian vegetation, killing bank-side trees, creating distinct vegetative patches and producing “beaver meadows” when abandoned dams are breached. Previous studies typically considered overall habitat impacts because dam age was usually unknown and channel characteristics prior to colonization were unavailable. We examined the impact of an ~9 month-old beaver dam on transport-retention using upstream-downstream comparisons, ammonium-tracer injections and comparisons with pre-impoundment data. The new beaver dam increased mean width from 4.4±1.2 m to 17.6± 3.9 m. Mean channel depth increased from 20±6 cm. near the face of the dam to 110±15 cm. tapering to normal 180-200 m upstream. The dam effectively reduced SPOM load from 3.1± 0.05 mg/L upstream to 1.1±0.05 mg/L at the dam under base flow conditions. In a former scour pool ~160m upstream from the dam, SPOM accumulated to a depth of ~25 cm. Pore water ammonium (10 cm-depth) increased from 25-95 µg-N/L (before) to 11240 µg-N/L, presumably from ammonification of deposited POM. A bromide tracer- ammonium injection study indicated ammonium retention in both the beaver pond and an equivalent length of free flowing channel downstream. However, comparing tracer dilution relative to ammonium disappearance indicates that most ammonium retention occurred behind the beaver dam. New beaver impoundments actively store and transform dissolved and particulate nutrient phases which should result in long-term retention of nutrients from the adjacent terrestrial landscape.