SYNTHESIS OF OHWM FEATURES

ERDC-TR-04-1
Page Navigation
12

1.7

magnitude, duration, frequency, and timing of flows and the development of

physical characteristics of arid-land stream channels. Table 1 summarizes the

geomorphic and vegetation features associated with the OHWM in arid streams.

The authors found little support from the literature for sub-regional distinctions

in these indicators across the arid Southwest. Therefore, they are presented as a

unified regional list, pending further discussion and testing.

Potential OHWM indicators are sorted into several categories. As pointed out

by Field (Chapter 3), the attempt to identify useful OHWM indicators should

include "test positive" and "test negative" sets of indicators. So Table 1 presents

OHWM indicators located above, at, and below the OHW boundary. In addition,

vegetation indicators are sorted into the riparian wetness classes mentioned

earlier--hydroriparian, mesoriparian, and xeroriparian. Hydroriparian vegetation

is present in areas that are perennially saturated or inundated; mesoriparian

communities are in areas that are seasonally moist; and xeroriparian communities

are in areas that are mostly dry with infrequent flood events. Based on the

editor's (RWL) experience, a few of the vegetation OHWM indicators in Table 1

are listed in vegetation categories beyond where they were described and

referenced in Chapter 4.

Features listed in Table 1 serve to divide the riparian area into zones

extending laterally from the deepest part of the main channel. The strongest

evidence of OHW are those features found consistently below the OHWM, such

as crested ripples, meander bars, and hydromesic ruderal herbs. Many of these

features are associated with the bottom of the channel. In contrast, strong

evidence of nonjurisdictional conditions is represented by those features that are

consistently above the OHWM, such as desert pavement, developed soils, and

upland plant species. Together these two zones bracket a central zone within

which the OHWM is located. The challenge lies in identifying a consistent and

reliable OHWM within this zone.

The relationship between physical features and "ordinary" hydrologic events

is difficult to establish in arid stream channels because of the extreme climatic

variability that is typical of the region. Variability is also the defining

characteristic of stream flows in dry-land intermittent and ephemeral stream

systems. Thus, the concept of ordinary in this region must include a range of

flows from the average annual or biennial high flow to some more extreme event.

The occurrence of physical features resulting from both types of events may