12.3: Soil, Streams and Agriculture

Soil, Streams, and Agriculture

Most of the surface of the Great Valley is covered by recent and Pleistocene-age alluvium that has been shed off the Sierra Nevada on the west side of the valley, as well as smaller volumes of sediment eroded from the Coastal and Temblor Ranges. Streams serve as pathways for sediment transportation from the mountains to the valley floor, where they are deposited. As a stream loses energy, it becomes less capable of carrying sediment and deposits it onto the broad, flat floodplains of the Great Valley. These sediments, characterized by distinct clay-rich mineralogy, contribute to the fertility of the agricultural land. The fine-grained alluvium is periodically replenished by spring runoff, making it one of California's most valuable natural resources.

This model of deposition follows the concept of the three zones of a river (Figure 12.4.1). Each zone is based on the dominant process that is operating in that part of the river system and include source, transport and sink. Source indicates erosion from steep slopes and migrating river channels that mechanically weather existing rock. The sediment that is transported includes sand, mud and pebbles that have been abraded, or mechanical weathered through physical forces of rock rubbing and pounding against rock within the river. Transport involves the movement of the weathered materials down slope. Sink is where the transportation energy runs out due to lack of slope, energy in the river system, and/or density of the sediment being transported. Sediments are deposited river deltas and floodplains, very much like those that exist within the Great Valley.

Figure \(\PageIndex{1}\): Based on the dominant processes operating within a river system, three zones may be identified. "Sediment Transport in Streams: Source to Sink" by Pacific Coast and Marine Science Center and the USGS is in the public domain. Access a detailed description.

Periodic glaciation of the Sierra Nevada has had significant impacts on the landscape as well. The effects of global climate change have also strongly influenced the rivers within the Great Valley. During glaciation periods, large amounts of sediment accumulated in moraines at the edges of alpine glaciers. Sierra Nevada rivers, fueled by abundant glacial meltwater and a considerably wetter climate during those times, transported most of this sediment downstream to the Sierra foothills and into the Great Valley. Geologists refer to these episodic alluvial deposits as glacial outwash. Over the past 2 million years, at least four major pulses of deposition occurred as sediment washed out from the Sierra Nevada during glacial episodes.

In the central to lower parts of the Great Valley, younger alluvium has buried older deposits, which geologists have identified through drill cores obtained from beneath the valley floor. Much of the understanding of sediment characteristics stems from early and recent petroleum exploration efforts. Currently, the floodplains of the Sacramento and San Joaquin Rivers and their tributaries receive sediment during times of river overflow, further burying the older layers of sediment and soils. However, along the Sierra foothills, the older alluvium can be observed at the surface. Due to the ongoing uplift of the Sierra Nevada relative to the Great Valley, rivers continuously cut downward into the landscape (Figure 12.4.2). Blue traces in Figure 12.4.2 represent some but not all of the major rivers found flowing within the Great Valley. The transparent blue is an outline of the Great Valley from Redding in the north to Bakersfield in the south.This cutting action leads to the formation of stream terraces, with remnants of older floodplain surfaces know as stream terraces (Figure 12.4.3). Note that each T marks a different time with T1 standing higher than existing younger terraces (T3). In this example, the present floodplain (T4) will evolve to form a new terrace as tectonic uplift and river incision continues. Stream terraces can be identified by their flat surfaces (Figure 12.4.4). The Merced River comes out of the foothills and onto the Great Valley floor approximately 48 km (30 miles) east of Turlock, CA in Figure 12.4.4. This particular butte the exhibits sandstone and kaolinitic clay is a great example of a river terrace in the Great Valley. Along the edges of the main river valleys, multiple terrace levels have been preserved at various elevations above the modern rivers. These terrace levels provide a record of the sequential downcutting of the rivers over time. As the elevation above the modern rivers increases, older valleys and deposits can be found.

Figure \(\PageIndex{2}\): Major rivers of the Great Valley Province. “Great Valley Province Major Rivers” by Cole Heap, P.G. is licensed under CC BY-NC 4.0. Constructed in a GIS using data from California Open Data Portal. Access a detailed description.

Figure \(\PageIndex{3}\): A series of terraces along a river. By Sushant Savla CC BY-NC 4.0. Access a detailed description.

Figure \(\PageIndex{4}\): Ione formation river terrace near Merced Falls in the Sierra Nevada foothills. "Ione Formation" by California Division of Mines and Geology licensed under public domain. Access a detailed description.

Query \(\PageIndex{1}\)

References

1. Arkley, R. J. (1954). Soils of eastern Merced County, California: California Division of Mines and Geology Bulletin. 182
2. Bertoldi, G. L., Johnston, R. H., & Evenson, K. D. (1991). Ground Water in the Great Valley, California - A Summary Report. U.S. Geological Survey Professional Paper, 1401-A.
3. California Department of Food and Agriculture (2010). California Agriculture Statistical Report 2008-2009.
4. Davis, G. H., Logren, B. E., & Mack, S. (1964). Use of ground-water reservoirs for storage of surface water in the San Joaquin Valley, California. U.S. Geological Survey Water-Supply, Paper 1618.
5. USGS. https://www.usgs.gov/media/images/sediment-diagram-source-sink. Accessed July 30, 2023