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26 BIOLOGICAL REPORT 19 Contrary to Green's assertion (Green 1935), the St. Croix Falls, which was only a series of rapids, was not a physical banier to fish and mussel move- ments within the St. Croix River (Upham 1900; Underhill 1957). However, the St. Croix Falls Dam, constructed 84 years ago, has been a barrier, and it will be used in this paper to delineate the upper from the lower St. Croix basin. There are an addi- tionall34 dams on tributary streams in the basin, but their effects on faunal movements have not been assessed. Within the entire basin there are 1,770 tributary streams to the St. Croix River (9,004 km total length), of which 1,077 are in Minnesota. Ninety- eight of these streams drain directly into the St. Croix mainstem, but most drain into the St. Croix's six major tributaries (Snake, Kettle, Namekagon, Apple, Clam, Yellow rivers). Minnesota has a greater number of streams, but the total length of streams in each state is about the same (Table 1). All of the major tributaries, except the Apple River, drain into the St. Croix River upstream of the Taylors Falls Dam. The Sunrise River, the major tributary in Chisago County, Minnesota, also en- ters the mainstem upstream of the dam. The Apple River and two smaller Wisconsin tributaries, the Willow and Kinnickinnick rivers, enter down- stream of the dam. On the St. Croix River, or connected directly to a tributary stream, are 628 lakes (~ 0.4 ha surface area) that have a total surface area of 41,012 ha. An additional 1, 725 lakes that do not have a surface water connection to the St. Croix River occur within the basin (Table 2). The Wisconsin portion of the St. Croix River basin contains 82010 of the open lakes and 78010 of the closed lakes in the basin. These lakes account for 67010 of the total lake area of the basin. The St. Croix River flows through three of the ecoregions described by Omernik and Gallant (1988). Nearly 58010 of the basin is in the Northern Lakes and Forests ecoregion, a region charac- terized by conifer and hardwood stands with exten- sive areas of wetlands and lakes; 39010 is in the Central Hardwoods Forests ecoregion, which is characterized by northern deciduous hardwood species; and 3010 is in the Western Corn Belt. Geology The bedrock of the upper St. Croix basin includes a large central area of late Precambrian basalt, flanked on the west by late Precambrian granite and sandstones, and on the east and south by Cambrian sandstones and shale (Helgesen et al. 1973; Lindholm et al. 1974a; Mudrey et al. 1982). Except for a finger of Precambrian basalt that extends southward to Taylors Falls, Minnesota, all of the bedrock of the lower St. Croix basin is Cam- brian and Ordovician sandstones and dolomite (Young and Hindall 1973; Lindholm. et al. 1974b; Morey 1976; Mudrey et al. 1982). Nearly all of the bedrock is overlain with glacial till and outwash, which is more than 70 m thick in many of the bedrock valleys. The lithologic composition of gla- cial deposits defies geographic summary, but in general heavy metal content (especially iron and manganese) tends to decrease and carbonate con- tent tends to increase from north to south. Along the river valley itself, the bedrock types mentioned above have been deeply eroded and sub- sequently fractured, leaving behind spectacular fa- cades that contribute greatly to the scenic value of the river. The rather wide and deep valley, strik- ingly noticeable in the lower St. Croix, was exca- vated by torrential flows of glacial meltwater that continued for 1,400-1,500 years (Clayton 1983; Hobbs 1983; Farrand and Drexler 1985). At Tay- lor's Falls, Minnesota, where the water encoun- tered resistant basaltic bedrock, the steeped-sided St. Croix Dalles was formed. At the river's confluence with the Mississippi River, a O.8-km spur of land from the Minnesota side narrows the river channel to 0.2 km. The wide floodplain established by Glacial River St. Croix permits maintenance of the spur by allowing the water to spread out behind it over the inner flood- plain, thus forming Lake St. Croix (Schwartz and Thiel 1954). The lake level also has been influenced since 1935 by the operation of Mississippi River Lock and Dam 3 near Redwing, Minnesota, which backs up Mississippi River water (0.2 m at the normal high water mark) into Lake St. Croix (K. Willis, U.S. Army Corps of Engineers, St. Paul, Minnesota, personal communication). Hydrology and Streamflow We used discharge data collected by the U. S. Geological Survey at six gaging stations to charac- terize streamflow within the basin (Table 3). Hy- drographs for the six gaging stations revealed fairly stable annual discharges over periods that varied from 23 to 88 years (Figs. 2 and 3). The Namekagon River and the Wisconsin portion of the St. Croix River showed less fluctuation around the average