Stream terrace

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Remants of former floodplains of the San Juan River in Utah stand as stream terraces above the river's modern level in this 1927 photo.

Stream terraces are features, often floodplains, left behind from periods when a stream was flowing at a higher elevation and has downcut to a lower elevation. Stream terraces often appear as plateaus on existing valley walls and indicate former elevations. Changes in elevation can be due to changes in the base level (elevation of the lowest point in the stream system, usually the drainage basin) of the stream system which leads to headward erosion along the length of the stream, gradually lowering its elevation. For example, downcutting by a river can lead to increased velocity of its smaller tributaries, causing them to erode toward their headwaters. Terraces can also be left behind when the volume of stream flow declines due to changes in climate, typical of areas which were covered by ice during periods of glaciation, and their adjacent drainage basins.

Unpaired stream terraces on the South Fork of the Shoshone River, Park County, Wyoming, 1923. The river at left has encountered a formation of erosion-resistant volcanic breccia, causing it to downcut more rapidly on the right, leaving terraces of different elevations.

Terraces of the same elevation on opposite sides of a stream are called paired terraces. They occur when a stream downcuts evenly on both sides. Unpaired terraces occur when the stream encounters material on one side that resists erosion, leaving a single terrace with no corresponding terrace on the resistant side.

[edit] References

• Leet, L. Don, Judson, Sheldon, and Kauffman, Marvin E., Physical Geology, 6th Edition. Englewood Cliffs, NJ: Prentice-Hall, 1982, pp. 283-285. ISBN 0-13-669762-3