The Kennebago River is in western Maine near Rangeley. It is surrounded by granitic mountains modified by continental glaciation. Its valleys are filled with glacial outwash, a material of mostly gravel and sand. These photographs were taken by Claude Epstein in July, 2001 of the river between Little and Big Kennebago Lake.
The channels of the Kennebago River between Little and Big Kennebago Lake are relatively straight. They have become entrenched beneath the glacial outwash that makes up the lowlands surrounding the channel. It is a G4 stream type following Rosgen's terminology.
The soils that form from the glacial outwash are gravelly sands with small percentages of finer material. When this soil erodes, the fine material, followed by the sand, is transported downstream, leaving the gravel to make up most of the channel material.
The steep banks of the Kennebago River are stabilized by forest vegetation. The next three photographs show different banks so stabilized. Though the last photograph is this series shows exposed gravel banks due to the construction of power lines just beyond the frame of this photograph.
Cutbanks, also called undercut slopes, occur where the vegetation is absent. The river during its flood stage erodes these banks forming vertical cliffs approximately 15 feet high. The exposed gravelly sand deposits are ancient flood deposits laid down by glacial meltwater streams.
Much of the eroded bank material is laid down along the margins of the stream channel. These are called gravel bars, or perhaps side channel bars. With each passing flood, more gravel may be added to these bars causing them to grow toward the middle of the channel. (Once in the middle of the channel, they are called mid channel bars though these are not depicted in the next three photographs.)
The next two photographs shows signs of human impact along the Kennebago River's channel. The first photograph shows the development of an alluvial fan at the base of a gully. The gully was formed by people climbing up and down the bank at this point. The sediment eroded by this traffic is depoisted at the base of the bank where the finer material was washed away by floods, leaving the gravel behind to form the alluvial fan.
People walk and drive down to the river banks along the edges of the bridge that crosses the Kennebago River at this point. This induces erosion, creating an alluvial fan from the coarser material. The alluvial fan, prograding into the channel, induces further gravel deposition from materials transported in the channel itself. These additional deposits are bars, added to the alluvial fan.
The channel bottom is almost all gravel. In this photograph, the water is swift and keeps the channel free of sand and mud. Only the hardiest aquatic vegetation roots in this setting.
Where the water flows more slowly, algae grow on the gravel surfaces trapping a covering of mud.
In addition, aquatic insect larvae, such as caddis flies, occupy the calmer water beneath the larger gravel fragments and may, as in this case, construct food-catching nets that trap organic detritus.
