A DRAINAGE NETWORK ESTIMATE OF SEDIMENT AND PARTICULATE ORGANIC MATTER STORAGE BY INSTREAM WOOD IN THE HEADWATERS OF NORTH SAINT VRAIN CREEK

We examined particulate organic matter (POM) and sediment (sand to cobble) storage by instream wood at fifty sites on the headwaters of North Saint Vrain Creek (NSV) in Rocky Mountain National Park. Over our 80 km² study area, there is a mix of confined and unconfined channel segments and a mosaic of subalpine forest ages, with both old-growth forest and more recently burned stands. Reservoirs of sediment and carbon are important for channel stability and biogeochemical cycling, respectively. Although wood and sediment have been studied on NSV, the magnitude of storage has not been quantified on reach and drainage basin scales, especially in relation to controls such as channel confinement, drainage area, and in-channel wood load. At each of the fifty sites, channel width, valley width, and reach length were delineated using a laser rangefinder. Wood load was calculated by approximating the volume of both logjams and dispersed wood pieces (treating individual logs as cylinders). POM and fine and coarse sediment were measured in the backwaters formed by jams and individual pieces. Preliminary results suggest that the cumulative sediment storage in first order tributaries is important, amounting to roughly a quarter of total network scale storage. On a reach scale, POM retention appears to be more correlated to surrounding basal forest age, whereas sediment storage appears to peak in unconfined, second order reaches with drainage areas on the order of 10-15 km². Although these findings suggest a majority of storage in larger (second order and above) channels, they also illuminate the cumulative importance of small, first-order channels in wood-related sediment storage throughout a drainage network.