WHERE THE LOGJAMS ARE: RESULTS FROM A DECADE OF LOGJAM SURVEYS IN ROCKY MOUNTAIN NATIONAL PARK, COLORADO (Invited Presentation)

Logjams that span the bankfull channel can store substantial quantities of sediment and organic matter and provide important aquatic habitat, as well as enhance lateral and vertical connectivity within the river corridor. Eleven annual surveys of 300 channel-spanning logjams on five mountain streams in Rocky Mountain National Park, Colorado, USA reveal consistent spatial and temporal patterns in the distribution of these jams. The number of logjams per unit length of channel correlates significantly and positively with the ratio of floodplain width to channel width, wood piece length to channel width, and the presence of multi-channel planform. More than half of the jams form around ramp and bridge key pieces. These patterns persisted throughout the period of study, despite significant variations in annual peak flow and recruitment of large wood via blowdown. Individual logjams typically last less than 10 years, but the average number of logjams per length of channel did not change significantly over 10 years. Individual logjams can undergo substantial interannual variation in their ability to alter downstream fluxes of material. These observations indicate that naturally formed logjams are dynamic features that tend to concentrate in wide, low gradient portions of a river network. This implies that river management designed to retain and reintroduce large wood should focus on reaches naturally retentive of channel-spanning logjams and long-term number of jams per unit length rather than the stability of individual jams.