COMPARISON OF CHANNEL MORPHOLOGY IN TWO ATLANTIC COASTAL RIVERS: PRELIMINARY RESULTS AND ANALYSIS

Bed sediment mobility is important to Atlantic salmon (Salmo salar) spawning and rearing success. Channel shape is an important factor affecting bedload mobility in streams. It is believed that channel morphology in many Maine rivers has been altered by land use practices, ultimately creating wider and shallower channels, and lowering stream competence. If correct, these changes may be partially responsible for the limited number of returning Atlantic salmon currently observed in Maine rivers. To evaluate the magnitude of these changes, we are preparing a statistical comparison of channel morphology between two Atlantic coastal streams: the Narraguagus River in Downeast Maine and the Jacquet River in northern New Brunswick, Canada. Compared to the Narraguagus, the Jacquet River has relatively healthy returns of adult salmon, and a differing land-use history. Both watersheds have similar drainage areas (Narraguagus 588 km²; Jacquet 510 km²), roughly equal mean annual precipitation (1244 mm; 1200 mm), but differing average channel gradients (0.16%; 0.51%). During the summer of 2007, we surveyed a 13.6-km section of the Narraguagus River with a drainage area range of 129-247 km², and a 10.4-km section of the Jacquet River with a drainage area range of 94-265 km². We made measurements of active and bankfull width and depth, and channel gradient at 100-m intervals, and performed grain-size counts at 200-m intervals. For the Narraguagus, our calculations use channel gradient data from topographic maps, because field measurements are imprecise in low-slope rivers. Preliminary calculations indicate significantly greater bankfull width-to-depth ratios on the Narraguagus River (30 ± 12) than in the Jacquet River (24 ± 8). Grain-size analyses yield a mean D₈₄ of 66 mm on the Narraguagus River and 89 mm on the Jacquet River. The unitless bankfull Shields parameter averages 0.025 on the Narraguagus River and 0.037 on the Jacquet River. These preliminary results are consistent with lower bedload mobility on the Narraguagus River.