Influence of sediment supply, lithology, and wood debris on the distribution of bedrock and alluvial channels

Field surveys in the Willapa River basin, Washington State, indicate that t he drainage area-channel slope threshold describing the distribution of bed rock and alluvial channels is influenced by the underlying lithology and th at local variations in sediment supply can overwhelm basinwide trends. Fiel d data from 90 short-reach surveys indicate that about one-eighth of the su rveyed reaches do not conform to a threshold defined by data from free-form ed alluvial and bedrock reaches due to the effects of logjams or local sedi ment sources or sinks. Mapping of channel type distributions in 18 extended reconnaissance surveys of >100 channel widths in channel length show that similar to 75% of the channel network was alluvial, but that the proportion of forced alluvial channels varies from 0% to 84%, Using the drainage area -slope thresholds defined by bedrock and alluvial data from the short-reach surveys, only 40% of the total channel length mapped in the longer reconna issance surveys was correctly classified from a 10 m grid digital elevation model. Of the misclassified reaches, 80% of the alluvial channels predicte d to be bedrock had forced alluvial morphologies, while almost half of the bedrock channels predicted to be alluvial were forced by low sediment suppl y, typically due to their location immediately downstream of large channel- spanning logjams, Poor representation of reach-scale slope in the digital t opography and/or a stochastic influence of sediment wave propagation likely account for the remaining misclassified channels, which together compose 7 % of the total surveyed channel length. Although variations in sediment sup ply can locally overwhelm the channel type predicted by the threshold model , the effect of logjams masks any influence of propagating sediment waves o n the distribution of bedrock and alluvial channels in the Willapa River ba sin.