Chapter 1. Introduction
SYNTHESIS OF OHWM FEATURES
The literature reviews on hydrology (Chapter 2), geomorphology (Chapter
3), and vegetation (Chapter 4) explore in detail the connections between the
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