A GIS ANALYSIS OF BROADSCALE GEOMORPHIC AND ECO-GEOMORPHIC CONTROLS ON ATLANTIC SALMON HABITAT IN DOWNEAST MAINE

Eastern Atlantic salmon populations have undergone drastic reductions in the last thirty years potentially linked to overfishing, stream barrier construction, and large-scale deforestation/land use changes over the 20th Century. Using a vast database collected by the Gulf of Maine Coastal Program, in combination with field validations, we analyzed Downeast Maine watersheds to study the relationship between salmon habitat, surficial geology, channel morphology, and large woody debris (LWD). The first part of our analysis captures broadscale geomorphic controls on both spawning and rearing habitat and we then focused a more detailed eco-geomorphic analysis on the Ducktrap River, which is ~16 km long draining ~ 39 square km. The channel morphology is characterized by 46% flatwater, 14% runs, 39% riffles, and 0.6% falls with rearing habitat comprising 100% of all riffles and runs and spawning habitat comprising 23% and 40% of riffles and runs respectively. The river basin is dominated by till (86%) and lesser glaciomarine fine-grained facies deposits (14%) with 94% of rearing and 89% of spawning area occurring in areas dominated by till. LWD frequency in the river is 100 logs per km with a total of 1567 logs measured (with >10cm diameter): 797 logs have diameters between 10 to 20 cm (small), 751 logs have diameters between 20 to 50 cm (medium), and 19 logs with diameters >50 cm (large). 31% of logs are oriented parallel, 19% perpendicular, 36% downstream, and 14% upstream where 230 of these logs perform some sediment storage function. Of the 230 sediment storing LWD, 17% are oriented parallel, 31% perpendicular, 34% downstream, and 18% upstream. These results suggest that there is a dearth of LWD in the Ducktrap and other Downeast Maine systems, where a lack of large LWD (19 pieces total) and nearly ubiquitous high decay class indicate relative disequilibrium. This imbalance is attributed to 20th Century land use/changes whereby riparian forests were cut and are now in a state of transitional growth reducing the recruitment and stability of LWD in the channel and adversely affecting salmon habitat in the river.