Spatial trends in large woody debris (LWD) storage and transport on the Lower Roanoke River, North Carolina

Background/Question/Methods

Large woody debris (LWD) is essential aquatic habitat in fine-grained low-gradient rivers.  Unfortunately, there is a paucity of large-river LWD data for the Eastern USA, especially in the low relief silt/clay and fine sand dominated Coastal Plain province.  The purpose of our study is to quantify the LWD population and determine the processes involved in LWD transport for the Coastal Plain segment of the Roanoke River (river), North Carolina.  A census was completed along a 177 km reach using georeferenced aerial video taken in March 2007.  The entire LWD population of the North bank and 20 one mile (1.6 km) representative reaches of the South bank were determined.  LWD transport is currently being quantified using radio frequency identification (RFID) tags read by both fixed station and mobile readers. 

Results/Conclusions

The population of the North bank consisted of 5041 individual detached LWD, 5266 additional trunks stored in log jams and 3306 trunks identified as future LWD, either as standing snags or  trees with exposed root balls.  Of the LWD available for transport (75% of population) approximately half are stored in jams, consisting of 3 or more intertwined trunks.  The majority of the 927 jams recorded are located in an actively widening reach with the lowest width depth ratios of the entire lower river.  The largest jams, however, form on bridge pilings where they are composed of at least 60 trunks.  Most jams (70%) are available nearly year round as aquatic habitat, positioned either on the lower bank or submerged at low-water flows.   Individual LWD, however, are generally on the upper bank with only 18% of the population in the channel during low water flows. The distribution of individual LWD is highly variable based on channel morphology and distance from the dam with an overall mean of 57 trunks per river km (both banks).  Normal 0 false false false MicrosoftInternetExplorer4 Preliminary RFID results indicate that LWD movement occurs primarily during the rising limb and crest of the hydrograph.  Also, individual trunks that move beyond one meander bend accumulate in log jams, at times several kilometers downstream from where they were last located.  The study is ongoing; we expect to link LWD distribution with other biotic studies along the river in the future.