Coffee Creek Stream Capture Geologic Area
Coffee Creek Stream capture from the northwest.
The Coffee Creek Stream Capture occupies approximately 200 acres in the Trinity Alps Wilderness (Salmon River District) about 12 air miles ESE of Cecilville. It is a textbook example of stream capture first described by Geologist Oscar H. Hershey in 1903. The upper 5 miles of Coffee Creek were captured by the Salmon River system. The capture was made possible by glacial moraines in the Coffee Creek Valley, and easterly headward erosion by the South Fork of the Salmon River.
Geologic Background
The stream capture likely occurred shortly after the first stage of glaciation in the Klamath Mountains, about 150,000 years ago, when the glacier in Coffee Creek Valley was high enough to permit a glacial meltwater stream to flow from the divide into the South Fork. The South Fork cut through the gap below the base level of Coffee Creek Valley capturing the headwaters of Coffee Creek, thus creating the stream capture. Today, lateral moraines from numerous glaciations are left high on the valley walls and recessional moraines can be seen in the valley as the forested areas between meadows.
Geologic Processes at Work
A geomorphologic map illustrating the fluvial and glacial processes.
As the last glacier retreated out of Coffee Creek Valley nearly 10,000 years ago, it left a broad U–shaped valley floor which has now been further incised by the Upper South Fork Salmon River (orange colors in the figure below) creating a narrow V–shaped incision characteristic of fluvial processes. This incision continues today and can be seen in the development of new younger terraces in the valley (yellow color) as the South Fork responds to climate and geomorphic changes. The light yellow color is the active alluvium which is where the present incision is taking place. Moraines are prominent features throughout the Special Interest Area and can be see as the red lines in the figure (from Sharp, 1960). The moraines are mostly comprised of cobbles and boulders of light grey quartz diorite, an igneous rock with a black and white speckled appearance. They are part of the Caribou Mt. Pluton, a large body of granitic rock exposed in the headwaters of Coffee Creek.
Stream Capture
Stream capture, also known as river capture or stream piracy, occurs when a river or stream bed is diverted and the water flows down another channel. There are several reasons why this geomorphologic phenomenon can happen.
Erosion - When the water is diverted and erodes the land and creates a new pathway for the stream, as in headwater erosion.
Tectonics - Movements of the Earth cause the slope of the land to change and the stream diverted from its former course.
Natural Damming - When debris from a landslide or an ice sheet block the stream and change the course of the water.
A generalized map of Coffee Creek Stream capture.
References
Elder, Don, 2002. Introduction to Klamath Mountains Geology. Siskiyou Field Institute. p. 1–8
Hershey, Oscar H., 1900. Ancient Alpine Glaciers of the Sierra Costa Mountains. Journal of Geology, v. 8, p. 42–57.
Hershey, Oscar H., 1903. Some evidence of two Glacial Stages in the Klamath Mountains in California. The American Geologist, v. 31, January to June, p. 139–156.
Sharp, Robert P. 1960. Pleistocene Glaciation in the Trinity Alps of Northern California. American Journal of Science, v. 258, p. 305–340.