Flows and Transport in Rivers - Hydro-Environmental Analysis: Freshwater Environments - page 114

with data from two sites on the Guadalupe River, Texas, downstream from Canyon Dam on February
2, 1977, when lows were manipulated to provide a range of values (9-784 cfs). The environmental
low targets were as follows:

• During October-February: 40% of the median lows by month

• During March-September: 60% of the median lows by month

The 60% level was chosen to be more protective of the riverine ecosystem during the spring
and summer periods, which are considered most critical to the warmwater isheries found in Texas
(Bounds and Lyons 1979; NRC 2005).

4.6.2.1.3 Biology-Based Design Flows
The biology-based design low is based on irst identifying exceedances. Exceedances are when
unfavorable conditions occur, such as when a low is less than some speciied value. The exceedance
condition is also termed an excursion . The premise is that if these exceedances occur rarely, then
the population will have time to recover, or, conversely, if they occur frequently, then no recovery
may occur. The biology-based design low is then the lowest low that will not cause exceedances
to occur more often than allowed by some speciied average frequency. The allowable frequencies
of exceedance are speciied so that they are suficiently small, and far enough apart, that they will
result in relatively small stresses on the biological community (USEPA 1986).
The biology-based design lows are computed so that they may be compared to the national, site-
speciic, and efluent toxicity criteria: the criteria continuous concentration (CCC) and the criterion
maximum concentration (CMC; USEPA 1986). For comparison, the low is based on site-speciic
durations (i.e., averaging periods) and frequencies speciied in the aquatic life criteria (e.g., 1 day
and 3 years for the CMC and 4 days and 3 years for the CCC). So, for example, for the CMC, a 4B3
low is computed, where the 4B3 is a biology-based 4-day average low event, which occurs (on
average) once every 3 years. The computer program DFLOW (most recent version at the time of
this writing was version 3.1 released in 2006) is available from the U.S. EPA for the computation of
biology-based lows and X Q Y lows (e.g., 7Q10) (http://www.epa.gov/waterscience/models/dlow/).

4.6.2.1.4 IHA and RVA Approaches
The IHA approach, which was developed by the Nature Conservancy along with Brian Richter, is
to irst analyze a suite of parameters associated with the natural low regime, using daily low data
such as from the USGS, and then to analyze and interpret those parameters to determine whether
some impact has occurred or is occurring (a hydrologic alteration). The IHA approach includes a
suite of hydrologic parameters (Table 4.8) that are ecologically meaningful and serve to character-
ize the range of inter- and intra-annual low variations as characterized by

• Flow magnitude

• Return interval (or frequency)

• Duration and timing

• Rate of change

The IHA also calculates the parameters for ive different types of environmental low compo-
nents: low lows, extreme low lows, high-low pulses, small loods, and large loods. A total of 34
additional parameters are used to characterize the environmental lows (Nature Conservancy 2007).
The strategy is implemented in an IHA software package distributed by the Nature Conservancy
(2007), which can use historical data downloaded from the USGS as well as other data. The soft-
ware can be used to compare between speciic sites for which there are historical low data or to
compare contiguous periods of the low record to each other to determine if a hydrologic alteration
has occurred. These parameters can be analyzed using nonparametric or parametric statistics.