BANK STABILIZATION, RIPARIAN LAND USE AND THE DISTRIBUTION OF LARGE WOODY DEBRIS IN A REGULATED REACH OF THE UPPER MISSOURI RIVER, NORTH DAKOTA, USA

摘要

Large woody debris (LWD) is an important component of ecosystem structure and function in large floodplain rivers. We examined associations between LWD distribution and riparian land use, bank stabilization (e.g. riprap revetment), local channel geo-morphology, and distance downriver from the dam in the Garrison Reach, a regulated reach of the upper Missouri River in North Dakota, USA. We conducted a survey of shoreline-associated LWD in the reach during typical summer flow conditions. Reach-wide LWD density was 21.3 pieces km~(-1) of shoreline, of which most pieces (39%) were 'beached' between the waterline and the bankfull level, 31% of pieces had evidence of originating at their current location (anchored), 18% of pieces were in deep water ( > 1 m), and 13% were in shallow water. LWD density along unstabilized alluvial (sand/silt) shorelines (27.3 pieces km~(-1)) was much higher than along stabilized shorelines (7.2 pieces km~(-1)). LWD density along forested shorelines (40.1 pieces km~(-1)) was higher than along open (e.g. rangeland, crop land; 9.2 pieces km~(-1)) or developed (e.g. residential, industrial; 7.8 pieces km~(-1)) shorelines. LWD density was highest overall along unstabilized, forested shorelines (45 pieces km~(-1)) and lowest along open or developed shorelines stabilized with a blanket-rock revetment (5.5 pieces km~(-1)). Bank stabilization nearly eliminated the positive effect of riparian forest on LWD density. A predicted longitudinal increase in LWD density with distance from the dam was detected only for deep LWD (including snags) along unstabilized alluvial shorelines. Partial resurvey in the summer following the initial survey revealed a reduction in total LWD density in the reach that we attribute to an increase in summer flow between years. Changes in riparian management and land use could slow the loss of LWD-related ecosystem services. However, restoration of a natural LWD regime in the Missouri River would require naturalization of the hydrograph and modification of existing bank stabilization and channel engineering structures.