GEOLOGY, STREAM PIRACY AND THE TMDL AT WALKER BROOK

Sampling at Walker Brook in north-central Minnesota, showed that the reach was impaired for DO under the Clean Water Act and required a water restoration study. Sampling data suggested that the usual variables associated with DO were not wholly responsible for the deficit. Proxies for organic matter that usually bear a specific relationship to DO, were inconsistent. TKN for example, shows the usual inverse relationship to DO during times when streamflow is runoff dominated but during base flow conditions, TKN and DO have a direct relationship. Walker Brook is in a deeply incised glacial margin stream valley of Itasca age associated with a recessional moraine near Bagley MN. The valley floor is covered by calcareous fens, which together with groundwater from numerous springs on the south flank of the valley, apparently influence DO in stream base flow. Local Geologic History involves: 1) retreat of the Itasca Phase ice to a recessional moraine; 2) a marginal meltwater stream formed along the ice front that cut a deep ESE-WNW trending valley; 3) surging ice moved moraine deposits into the valley and onto its southern flanks; 4) debris dams caused ponding in the valley and diverted the upper reaches of the Clearwater northward into an abandoned tunnel valley; 5) after a period of lake deposition, Walker Brook breached one of the dams to become a tributary of the Clearwater River. In the modern hydrological setting, high hydraulic heads in the over-thrust moraine material on the southern flank of the valley, drive groundwater through the fens bordering the stream toward the north. Base flow recession is ~ 4 days so runoff peaks are short lived although water stored in the fens can support flow for some time after a rainfall event. This information will be used to reclassify Walker Brook in Minnesota's water quality scheme, thereby removing it from the impaired waters list.