Measures of the Health of Rivers and Streams - Hydro-Environmental Analysis: Freshwater Environments - page 185

7 Measures of the Health

of Rivers and Streams

7.1 INTRODUCTION
The stated objective of the Clean Water Act (CWA) is to “restore and maintain the physical, chemi-
cal and biological integrity of the Nation's waters” (United States Code title 33, sections 1251-1387).
In order to promulgate regulations to meet the objectives of the CWA, one challenge has been to
develop well deined and enforceable metrics and operational conditions that can be used to both
assess the condition of the nation's waters and determine when the objective of the CWA is met.
One dificulty is the scale of the problem and the diversity in waterbody types, habitats, and
aquatic organisms across the United States. For example, the major rivers and streams of the conter-
minous United States, as illustrated in Figure 7.1, comprise only 10% of the length of U.S. lowing
waters.
Changes occurring on an even larger scale resulting from physical and landscape changes are
illustrated by the river continuum concept (RCC) developed by Vannote et al. (1980) (Figure 7.2).
The RCC illustrates how watersheds and streams are connected and how variations in watershed
and stream characteristics impact biological communities from the headwaters to the mouth of
streams. For example, according to the RCC, productivity in low-order streams is more limited
due to shading by riparian vegetation, so that most of the energy coming into these systems is from
outside (allocthonous) sources, such as leaf fall. As a result, the habitat favors organisms that can
grow or feed on these coarse organic sources. Toward the mouth, rivers are wider, slower, and
deeper, and more of the productivity is derived from internal sources (autochthonous), such as
phytoplankton.
In addition to the complexity of the spatial distribution of organisms, variations in time are also
important. For example, often a particular life stage may be impacted by conditions at speciic times
of the year, such as the conditions for low and temperature.
A wide variety of methods have been developed and implemented to assess the “physical,
chemical and biological integrity” of our rivers and streams, a number of which are discussed
in the following text. Some of the most commonly used methods (chemical integrity) have been
based on (water quality criteria) establishing a designated use for a waterbody that is used to
develop goals for that waterbody, usually indicated by the concentrations of materials dissolved or
suspended in the water column and speciied in water standards. In addition to the concentrations
of materials in water, low and habitat metrics have been developed (physical integrity). Flow
criteria have ranged from setting minimum lows to establishing goals based on the natural low
regime (see Section 4.6.2, Chapter 4). The presence or absence and the numbers and distributions
of speciic organisms or groups of aquatic organisms, the indicator species, have also been used
to assess the biological integrity. Metrics have also included the distribution or diversity of organ-
isms within the biotic community as an indicator of biotic health or integrity. Finally, multimetric
methods (e.g., the biological condition gradient [BCG], the index of biotic integrity [IBI], and
the index of biological condition) have been developed (and will be introduced in this chapter)
to assess the “physical, chemical, and biological” conditions and the integrity of our rivers and
streams.