California Rapid Assessment Method for Wetlands - State Water ...

California Rapid Assessment Method for Wetlands v. 5.0.2 – Chapter 4
4.2 Attribute 2: Hydrology
Hydrology includes the sources, quantities, and movements of water, plus the quantities,
transport, and fates of water-borne materials, particularly sediment as bed load and suspended
load. Hydrology is the most important direct determinant of wetland functions (Mitch and
Gosselink 1993). The physical structure of a wetland is largely determined by the magnitude,
duration, and intensity of water movement. For example, substrate grain size, depth of wetland
sediments, and total organic carbon in sediments tend to be inversely correlated to duration of
inundation in a lacustrine wetland. The hydrology of a wetland directly affects many physical
processes, including nutrient cycling, sediment entrapment, and pollution filtration. For
example, Odum and Heywood (1978) found that leaves in freshwater depressional wetlands
decomposed more rapidly when submerged. The hydrology of a wetland constitutes a dynamic
habitat template for wetland plants and animals. For example, Richards et al. 2002 concluded
that meandering and braiding in riverine systems control habitat patch dynamics and ecosystem
turnover. The spatial distribution of plants and animals in a tidal marsh closely correspond to
patterns of tidal inundation or exposure (Sanderson et al. 2000).
4.2.1 Water Source
A. Definition: Water Sources directly affect the extent, duration, and frequency of saturated or
ponded conditions within an Assessment Area. Water Sources include inputs of water into the
AA as well as any diversions of water from the AA. Diversions are considered a water source
because they affect the ability of the AA to function as a source of water for other habitats while
also directly affecting the hydrology of the AA.
Inputs of water affecting conditions during the dry season are especially important because they
strongly influence the structure and composition of wetland plant and animal communities. The
Water Source metric therefore focuses on conditions that affect dry season hydrology.
Natural water sources include precipitation, ground water discharge, and flooding of the AA due
to high tides or naturally high riverine flows. Examples of unnatural sources include stormdrains
that empty into the AA or into an immediately adjacent area. For seeps and springs that occur at
the toes of earthen dams, the reservoirs behind the dams are water sources. Large reservoirs and
lakes that do not drain directly into the AA should not be considered water sources, although
they can have systemic, ubiquitous, effects on the condition of the AA. For example, although
the salinity regimes of some estuarine wetlands in San Francisco Bay are indirectly affected by
dams in the Sierra Nevada, others are directly affected by nearby discharges from sewage
treatment facilities. This metric focuses on the more direct water sources. Engineered
hydrological controls, such as pumps, weirs, and tide gates can serve to demarcate the boundary
of an AA (see Section 3.5), but they are not considered water sources.
B. Rationale: Wetlands depend on constant or recurrent, shallow inundation or saturation at or
near the surface of the substrate (National Research Council 2001). Consistent, natural inflows
of water to a wetland are important to their ability to perform and maintain most of their
intrinsic ecological, hydrological, and societal functions and services. The flow of water into a
wetland also affects its sedimentary processes, geo-chemistry, and basic physical structure. Sudol
and Ambrose (2002) found that one of the greatest causes of failed wetland mitigation or
restoration projects is inadequate or inappropriate hydrology.
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