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Weathering and Erosion
The Mojave River
Stream Terraces and Older Surfaces
Stream terraces form when streams carve downward into their floodplains, leaving discontinuous remnants of older floodplain surfaces as step-like benches along the sides of the valley. Stream terraces are common throughout the Western United States. In the context of this discussion on the Mojave region, older surfaces represent flattened areas (plateaus, mesa, uplands areas, hillside benches) that are stable or isolated, neither experiencing significant rates of sediment buildup (aggradation) or down cutting by erosion. These older surfaces may have no clear or obvious connection to a more modern drainage system in a particular area. Terraces and older surfaces preserve or display unique characteristic soil profiles or weathering characteristics because of their long-standing isolation from stream erosion.
Many factors influence why streams episodically carve into their floodplains, forming stream terraces. Because stream terraces are typically widely distributed along steams throughout a region, changing climatic conditions are likely a most important contributing factor to their formation. Streams broadened their floodplains when sediment supplies are high and down cutting by stream erosion is abated. In cool, wet periods, plants typically cover the landscape, and hence sediment supply is low; enhanced moisture increases stream flows, and streams draining mountainous regions will cut downward. During dry periods, plants don't provide enough cover to prevent intense erosion during infrequent storms. As a result, high sediment yields may result in the backfilling of stream channels. This natural feedback system is much more complex than this because many other processes occur simultaneously. Under cooler, wetter conditions during an ice age, soil development and weathering processes proceed faster due to more frequent wetting and drying, more freeze-thaw cycles, and increased biological activity (particularly root penetration). Soils formed during extended wet periods can be released as sediments once the groundcover is removed during drought conditions, especially by wildfire followed by a rainstorm.
Climate is also a factor in the development of caliche (calcium-carbonate-rich crusts or soils that form in desert conditions). In North America, caliche is found in arid or semiarid regions of the western states. In many places in the Mojave region these calium-carbonate-rich crusts form a resistant caprock along stream terraces.
Next > The Mojave River and Associated Lakes