FRESHWATER MUSSEL SURVEY OF THE UPPER MISSISSIPPI (DAYTON, MN. TO LOCK AND DAM 3), LOWER ST. CROIX, AND LOWER MINNESOTA RIVERS, 2000-01. Dan Kelner1 and Mike Davis2, Minnesota Department of Natural Resources, Division of Ecological Services. 1500 Lafayette Rd. St. Paul, MN 55155. 21801 S. Oak St. Lake City, MN 55041. In 1999, a survey was begun to determine the distribution and abundance of unionoid mussels in Minnesota. During 2000 and 2001, as part of this effort, 167 sites were sampled along an 83 mile (134 km) stretch of the Upper Mississippi River (UMR) that extends from approximately 20 miles (32 km) north of the Twin Cities near Dayton, MN. (RM 880), through the Twin Cities to Lock and Dam 3 near Red Wing, MN. (RM 797). Five pools or reaches were surveyed within this stretch and include from upstream to downstream; Coon Rapids Pool (above Coon Rapids Dam), St. Anthony Falls Pool (above St. Anthony Falls [SAF]), and Pools 1, 2, and 3. During 2001, 20 sites along a 24 mile (39 km) reach of the lower St. Croix River (LSCR) from Stillwater, MN to its confluence with the UMR at Prescott, WI, and 13 sites along a 4 mile (6 km) reach of the lower Minnesota River (LMNR) to its confluence with the UMR were also surveyed. Sample methods were consistent throughout the study and consisted of timed searches and hand collection of mussels while wading, snorkeling, and diving. One-person hour/site was targeted as the search time and sites were typically spaced no more than 1 mile (1.6 km) apart. Quantitative samples were also collected and mussel bed boundaries mapped at five sites within the UMR. Zebra mussel (Dreissena polymorpha) density was determined from quantitative samples and zebra mussels attached to unionids collected from timed searches were counted. Over 25,000 live mussels representing 30 species were collected with an additional 11 species collected as empty shells. A total of 27 live species were collected in the UMR proper, 25 in the LSCR, and 9 in the LMNR. Mussels in the LSCR were more abundant and the assemblage appeared to more closely support its historic compliment of mussel species as compared to the UMR and LMNR. Exclusive to the LSCR mussel assemblage was the federally endangered Lampsilis higginsi. The mussel fauna of UMR Pools 1, 2, 3 appear to be recolonizing since its reported decimation by pollution during the first half of the 1900's. The survey provided clear evidence of recent and ongoing recruitment; many of the individuals collected were less than 10 years old. Several state listed species were collected including two listed as endangered in Minnesota in fairly high numbers (Arcidens confragosus and Quadrula nodulata). Neither of these two species were collected in the LSCR or LMNR. Recolonization is probably due to improved water quality conditions over the past 15-20 years. Furthermore, mussels may be expanding their range above SAF, which historically served as a faunal barrier to upstream dispersal but now are circumnavigated by locks. A total of 16 live species were collected from the St. Anthony Falls Pool including 10 species previously not reported above SAF, and the community very closely resembles the communities of Pool 1 and upper Pool 2 in species composition. Zebra mussels were absent above SAF and nearly absent from UMR Pools 1-3 and LMNR (<0.1% unionids infested and density < 0.1/m2). Nearly 1% of the unionids in the LSCR were infested with zebra mussels, many of which were <10mm in length. These UMR pools differ from those downstream (Pool 4 and below) where zebra mussels are extremely abundant and are decimating the native mussel communities and from the LSCR where zebra mussels have recently invaded and appear to be reproducing. Ironically, this reach of the Mississippi River between the Twin Cities and Red Wing, MN., once nearly a dead zone, may now constitute one of the last big river mussel refuges in the Midwestern United States. Keywords: freshwater mussel, Missisippi River drainage, unionidae, Minnesota, Dreissena polymorpha