Large wood dynamics in disturbed riparian areas and associated impacts on channel morphology

Large wood (LW) is delivered to streams through a number of processes, ranging from chronic input from riparian and hillslope sources to large, episodic inputs from disturbances such as landslides. This study follows LW and channel dynamics associated with multiple watershed disturbances for nearly 20 years using repeat surveys, LW tagging, and reconnaissance in 2 streams in NW Wyoming. The initial disturbance was wildfire that burned 80% of the forest cover in one watershed in 2000, including forested riparian areas. Notably, little change in LW occurred during the first 3 years post-fire (2001-03) and the channel maintained a largely stable, plane bed form. Over time, as burned trees began to decay and fall, inputs from burned areas increased, essentially doubling and tripling the number of pieces of LW in some reaches. Observed changes measured during a re-survey in 2008 included: 1) increase in the size and number of LW jams; 2) deposition of sediment within LW jams; 3) channel avulsions; 4) erosion of banks and terraces; and 5) new sources of sediment from bank erosion. Monitoring continued as additional disturbances were manifest, including beaver damming (and failure), a second wildfire, snowmelt floods, reactivation of landslides and earthflows, and mountain pine beetle infestation. Each of these disturbances further increased or mobilized wood and sediment loads. The most extensive channel changes measured during a 2013 re-survey were directly linked to jamming of instream wood and beaver dams and included: 1) avulsion associated with sedimentation behind LW dams and 2) shifts to multi-thread channels. Reconnaissance in 2018-19 shows that the channels and floodplain continue to adjust to the suite of disturbances and some reaches were newly de-watered due to repositioning of channels within the floodplain. Disturbances may be linked in part to changes in climate and chronic instability may reflect a new norm in these watersheds.