From Incised Valleys to Coarse Alluvial Streams - North Shore of Lake Superior, Minnesota

Most streams flowing into Lake Superior on the North Shore of Minnesota are relatively short and, in geologic terms, very young. Their valleys were recently carved or "re-carved" in the moraine deposits of the late Wisconsin phase as level of Glacial Lake Duluth (a precursor of Lake Superior) dropped rapidly (about 8 to 9 thousand years ago) to near present levels. A bedrock ridge, approximately parallel to Lake Superior shoreline, separates some of the streams into two sections with strikingly different characteristics: in the upstream sections (an elevated plateau) the streams have small gradients, deep channels, and wide meanders, similar to low-land rivers, while in the downstream sections, which is the focus of this study, the streams are steep, confined by deep and wide valleys with boulder size bed material, similar to some of the mountain streams. In some cases, the bedrock is commonly exposed in the streambed or banks while in some other cases thick, coarse (cobble to block size) deposits cover most of the streambed. Poplar River is a prime example of the latter. Steep, shallow, with boulder size bed material protruding above the water surface, in mountain river terminology the downstream section of the Poplar River resembles the plane-bed type. It lacks defined banks and although the bottom of the valley is few times wider than the stream, there is no floodplain. The stream valley is essentially the only source of sediment, a bimodal glacial till consisting of mostly cobble to boulder particles in matrix of clay, silt and fine sand. Delivery mechanism is mostly through landslide, hillslope failure due stream lateral mobility, and, to a lesser extent, lateral ravines. While the stream has than enough energy to carry nearly all the fine sediment (only a small fraction is trapped into streambed) all the way downstream (into Lake Superior), threshold computations based on discharge data would suggest that the coarse particles are only rarely mobilized, during extreme (snowmelt) flows, placing Poplar River into "transport limited" category. It appears that after an initial fast valley incision phase, as more coarse sediment armored the valley bottom, the overall channel incision has slowed down to a quasi steady state. The channel continued eroded laterally into valley sides, triggering landslides and widening the valley bottom. Despite the large size of the dominant sediment the channel planform and dynamic shares some characteristics with alluvial streams: lateral mobility, migrating meanders, cut-offs, and avulsions.