Stream terraces are relict features, such as floodplains, 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 earlier stream 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 a tributaries, causing that tributary to erode toward its 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.
Fill terraces: Fill terraces are the result of an existing valley being filled with alluvium. The valley may fill with alluvium for many different reasons including: an influx in bed load due to glaciation or change in carrying capacity which causes the valley, that was down cut by the stream, to be filled in with material (Easterbrook). The stream will continue to deposit material until an equilibrium is reached and the stream can transport the material rather than deposit it. This equilibrium may last for a very short period, such as, after glaciation, or for a very long time if the conditions do not change. The fill terrace is created when the conditions change again and the stream starts to incise into the material that it deposited in the valley. Once this occurs benches composed completely of alluvium form on the sides of the valley. The upper most benches are the fill terraces. As the stream continues to cut down through the alluvium the fill terraces are left above the river channel (sometimes 100 m or more). The fill terrace is only the very highest terrace resulting from the depositional episode, if there are multiple terraces below the fill terrace these are called cut-in-terraces.
Cut-in-terraces: Cut-in-terraces are similar to the fill terraces mentioned above, but they are erosional in origin. Once the alluvium deposited in the valley has begun to errode and fill terraces form along the valley walls, cut-in-terraces may also form below the fill terraces. As the stream continues to incise into the material, multiple levels of terraces may form. The uppermost being the fill terraces and the remaining lower terraces are cut-in-terraces.
Nested fill terraces: Nested fill terraces are the result of the valley filling with alluvium, the alluvium being incised, and the valley filling again with material but to a lower level than before. The terrace that results for the second filling is a nested terrace because it has been “nested” into the original alluvium and created a terrace. These terraces are depositional in origin and may be able to be identified by a sudden change in alluvium characteristics such as finer material for example.
Strath terraces: Strath terraces are the result of the stream downcutting through bedrock. As the stream continues to downcut a period of valley widening may occur and expand the valley width. This may occur due to an equilibrium reached in the stream system resulting from: slowed or paused uplift, climate change, or a change in the bedrock type. Once downcutting continues the flattened valley bottom composed of bedrock (overlain with a possible thin layer of alluvium) is left above the stream channel. These bedrock terraces are the strath terraces and are erosional in nature.
Paired and unpaired terraces: 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.
Stream terraces can be used to measure the rate at which a stream is downcutting its valley. Using various dating methods, an age can be determined for the deposition of the terrace. Using the resulting date and the elevation above the current stream level and approximate average rate of downcutting can be determined.
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