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

California Rapid Assessment Method for Wetlands v. 5.0.2 – Chapter 3
3.2.2.1.1 Non-confined Riverine Sub-type
In non-confined riverine systems, the width of the valley across which the system can migrate
without encountering a hillside, high terrace, or other feature that is likely to resist migration is at
least twice the average bankfull width of the channel. Non-confined riverine systems typically
occur on alluvial fans, deltas in lakes, and along broad valleys. A channel can be confined by
substantial artificial levees if the average distance across the channel at bankfull is more than half
the distance between the levees. This assumes that the channel would not be allowed to migrate
past the levees, and that levee breaches will be repaired. Confinement is unrelated to the channel
entrenchment. Entrenched channels can be confined or non-confined (Figure 3.3).
3.2.2.1.2 Confined Riverine Sub-type
In confined riverine systems, the width of the valley across which the system can migrate
without encountering a hillside, terrace, or substantial man-made levee is less than twice the
average bankfull width of the channel. A channel can be confined by artificial levees and urban
development if the average distance across the channel at bankfull is more than half the distance
between the levees or more than half the width of the non-urbanized lands that border the
stream course. This assumes that the channel would not be allowed to migrate past the levees
and that levee breaches will be repaired. Confinement is unrelated to channel entrenchment.
Entrenched channels can be confined or non-confined (Figure 3.3).
3.2.2.2 Depressional Wetlands
Depressional wetlands exist in topographic lows that do not usually have outgoing surface
drainage except during extreme flood events or heavy rainfall. Precipitation is their main source
of water. Depressional wetlands can have distinct or indistinct boundaries. Many depressional
wetlands are seasonal, and some lack surface ponding or saturated conditions during dry years.
A complex of shallows and seasonally wet swales and depressions created by the slight
topographic relief of a vernal pool system is an example of an indistinct depressional wetland.
The margins of distinct depressional wetlands are relatively easy to discern in aerial photos and
in the field. Examples of distinct depressional wetlands include sag ponds, snowmelt ponds,
kettle-holes in moraines, cutoff ox-bows on floodplains, and water hazards on golf courses.
3.2.2.2.1 Vernal Pool Wetlands
Vernal pools are ephemeral wetlands that form in shallow depressions underlain by bedrock or
by an impervious, near-surface soil horizon (Witham 2006). These depressions fill with
rainwater and runoff during the winter and may remain inundated until spring or early summer,
sometimes filling and emptying repeatedly during the wet season (USFWS 1994). Vernal pools
undergo four distinct annual phases: (1) the wetting phase with the onset of the first rains; (2)
the aquatic phase when the peak rainfall and inundation occurs; (3) the drying phase when many
plants flower and produce seed and many animals disperse; and finally (4) the drought phase
when the soil dries and cracks, and the plants succumb to extreme dry conditions (Zedler 1987).
Vernal pools typically support a minimum of 30% cover of native plant species during the
aquatic or drying phase. Vernal pools in disturbed areas or ones that are subjected to abnormal
rainfall patterns might not meet this criterion due to invasion by non-native plants (USFWS
1994). If the wetland is mostly characteristic of a vernal pool but also has characteristics of other
kinds of wetlands, such that its classification as a vernal pool is not completely certain, then it
should be considered a vernal pool.
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