HYDRAULIC AND GEOMORPHIC EFFECTS OF LARGE WOODY DEBRIS ADDITIONS IN THE NARRAGUAGUS RIVER WATERSHED, COASTAL MAINE

Maine coastal rivers are important to current ecologic and geomorphic research because they host the last remaining runs of endangered anadromous Atlantic salmon in the U.S. Atlantic salmon populations have declined over the last few hundred years with returning adults decreasing from ~500,000 in the 1880s to ~1,000 in 2000, and presently occur only in Maine from the Kennebec River to the Canadian border. Recent restoration studies have added large woody debris (LWD) to small tributaries to improve salmon rearing habitat. We study the effects of LWD additions in early August 2008 on hydraulics and substrate in Baker Brook, a west-flowing tributary of the Narraguagus River. We focus on the treatment reach nearest the confluence with the Narraguagus River (Baker1). The study site is divided into two reaches, treatment (Baker1-T) and control (Baker1-C). These are further divided into four sub-reaches based on channel gradient (~1% in Baker1-C-Flat and Baker1-T-Flat; >2% in Baker1-C-Steep and Baker1-T-Steep). The control sub-reaches allow us to quantify uncertainty because the degree of observed post-treatment changes will not depend on added LWD; changes in the treatment sub-reaches must be larger than those in the control sub-reaches to be deemed significant. We seek to answer three research questions: (1) how much does mean velocity through the study sub-reaches change as a result of additions; (2) how much does hydraulic roughness change; and (3) does sediment storage and sorting result from the LWD additions? We measured reach-average velocities (U_reach) using the salt dilution method in May, July and August 2008. We use rating curves to compare the post-treatment to the pre-treatment U_reach-stage relationship. Preliminary results on treatment-related changes in U_reach are inconclusive because we do not yet have enough post-treatment velocity data collected at the same stage as the pre-treatment measurements. We quantified sub-reach substrate median grain size with intensive clast counts in July 2008; these will be repeated post-treatment in May 2009. Analysis of pre-treatment substrate size show that the flat sub-reaches have a finer substrate size (34-38 mm) than the steep sub-reaches (88-134 mm). Comparison analysis of data gathered in May-August 2008 with additional post-treatment data collected in May 2009 will allow us to assess the effects of added LWD in Baker Brook.