Tradition holds that Red Lake was named for its beautiful red sunsets, and the name first appeared on a 1755 map as "Lac Rouge," meaning Red Lake. After glaciers covering Minnesota melted, meltwater pooled in northwestern Minnesota and Canada because western Minnesota's Continental Divide blocked southward flow and northern ice blocked drainage to Hudson Bay, forming Glacial Lake Agassiz, a body of water larger than the Great Lakes. When the ice retreated north of Winnipeg, Manitoba, Glacial Lake Agassiz drained north, leaving a remarkably flat landscape in northwestern Minnesota. North and west of Red Lake, there is less than 10 feet of elevation change through most of the old lake basin, and because water drained so slowly, the Red Lake Peatland, or Big Bog, began forming about 3,000 years ago. The Big Bog is the largest continuous expanse of peatland in the northern contiguous United States, and Upper and Lower Red Lakes lie within the old Glacial Lake Agassiz lake bed, as do Lake of the Woods, Lake Winnipeg, and Lake Manitoba. Red Lake's shallow waters help make it a highly productive fishery because summer winds continually stir the lake, exposing phytoplankton to sunlight and supporting abundant food for zooplankton and fish such as walleye and northern pike. Bulrushes along the lakeshore protect the shore from waves, stabilize the shoreline, attract birds, provide food and nesting cover for waterfowl and muskrats, and serve as spawning and nursery habitat for fish. The lake's constant mixing also explains its turbidity, its lack of strong summer temperature layering, and the absence of a pronounced fall algae bloom. Wind and wave action can also create Langmuir circulation, seen as lines of foam on the surface, and strong winds can produce a seiche, a tide-like rise in water level followed by hours of sloshing back and forth.