The area in its contemporary (pre-1850s) state began to form about 10,000 years ago at the end of the last Ice Age. During the Ice Age global sea levels were about 300 ft lower than today, and the Delta region, was a river valley through which the continuation of several rivers flowed to the Pacific Ocean. When sea levels rose again, ocean water backed up into the Central Valley; the combination of the narrow strait and tidal action pushing inland dramatically slowed the current of these rivers and forced them to drop sediment. The early area was composed of shifting channels, sand dunes, alluvial fans and floodplains that underwent constant fluctuation because of rapidly rising seas – 1 inch per year. About 8,000 years ago, the rate of sea-level rise slackened, allowing wetland plants to take hold in the Delta, trapping sediment; the growth and decay of these plants began to form the vast peat deposits that make up the area and the Delta’s islands.
Peat (turf) is an accumulation of partially decayed vegetation or organic matter that is unique to natural areas called peatlands or mires. The peatland ecosystem is the most efficient carbon sink on the planet because peatland plants capture the CO2 which is naturally released from the peat maintaining an equilibrium. In natural peatlands the "annual rate of biomass production is greater than the rate of decomposition" but it takes "thousands of years for peatlands to develop the deposits of 1.5 to 2.3 m, which is the average depth of the boreal peatlands". One of the most common components is Sphagnum moss, although many other plants can contribute. Soils that contain mostly peat are known as a histosol. Peat forms in wetland conditions, where flooding obstructs flows of oxygen from the atmosphere, slowing rates of decomposition.
Immediately before large-scale human development, most of the area had saucer-like cross sections, with low natural levees flanking a marshy interior "bowl" that flooded intermittently with the seasons. The height of these natural levees, formed from overbank deposits of sediments deposited by annual river floods, and ranged from 1 ft above mean high tide in some places to 7 to 8 ft in others. An estimated 60 percent of the area flooded for up to two hours each day at high tide; during spring tides or river floods, it was not uncommon for the entire area to be under water.
The early levees were built of peat, and were highly susceptible to wind and water erosion. The Great Flood of 1862 obliterated much of the existing Delta infrastructure, forcing landowners to rebuild their levees higher and stronger. After the area was drained and the protective native vegetation stripped off, the land began to subside. Subsidence continues today because of oxidation, dewatering and wind erosion of the area’s peat soils. The surface of some places lose up to 3 inches of peaty soil per year.
The Cosumnes River is the only unregulated river on the western slopes of the Sierra Nevada. Because it remains undammed, it experiences the seasonal overbank flooding that was once a common feature of rivers in California's central valley. This flooding can create wetlands that can be large, like what you see, or small like vernal pools that are only a few square yards in size. The can be permanent like lakes or ponds, or like tidal pools and mud flats.
To log this cache, email me the answers to the following:
- Please list the GC# of this cache and the number in your party in the first line of your email.
- Does this ground around you appear to be made from peaty soil, or from the clay from silt and sand deposited by the river.
- Does the land around appear to be sunken, or subsided, or does it appear to be at the same level as the nearby slough and river?
- Looking at the lake in front of you, was it created by a levee system or by seasonal flooding?
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If you feel like doing so, please post your picture in your log, but please do not include the sign.
More information about the Cosumnes River Preserve is available at www.cosumnes.org.