Flows and Transport in Rivers - Hydro-Environmental Analysis: Freshwater Environments - page 109

The emphasis of this type of management was often more focused on how much water could be
taken from a river rather than how much water is needed to be left in. Also, one major law of the
“low-low” approach was that there were only two relative conditions with respect to ish habitats for
consideration by resource managers: “the level below which disaster would occur, and everything
else” (Waddle 2001). In addition, for isheries management, low lows were often established for
other uses, such as water supply, navigation, etc. Low lows protective of aquatic life were some-
times referred to as environmental lows, which is not consistent with what today would be consid-
ered to be an environmental condition.
Today, in practice, while the term minimum lows is often used, particularly for regulatory pur-
poses, today's minimum low requirements are more commonly based on “instream low uses,” or
how much water do we need to leave as opposed to how much water can we take out. For example,
Waddle (2001) stated that with instream lows driving low-low criteria, “biologists no longer were
trying to ind that magical low level below which a stream should not be dewatered. Instead, they
were in a position to assert instream low needs for ish habitats and other environmental values.
Furthermore, they could do so in terms of the seasonal life cycle needs of the ish (or other aquatic
organisms) over the annual hydrograph.”
The minimum condition may refer to instream lows (lows within the stream), and is also referred
to as minimum instream low requirements. The minimum condition may also refer to water levels.
In Florida, for example, state statutes require that the state's water management districts establish
minimum lows and levels (MFLs). Minimum lows are for rivers and streams, while minimum
levels are for lakes and aquifers. Each management district develops MFLs for its waterbodies and
reports their status annually to the state. The objective of the MFLs is to protect the waters of the
state from signiicant harm due to permitted withdrawals. While the MFLs may be a magnitude,
for some management districts, such as the St. Johns River Water Management District, they also
include a frequency component (Figure 4.28).
As discussed previously, the 7Q10 low is probably the most commonly used low index in the
United States. The 7Q10 low is most often used as a “reasonable worst case” low-low condition for
allocating waste loads, but it has also been used in a number of other regulatory and management
contexts, such as in an analysis of chronic toxicity, as an indicator of drought conditions, in the regu-
lation of withdrawals, and for numerous other purposes as described by Pyrce (2004). In addition to
the 7Q10 low, other 7Q X lows have been used for other purposes, such as the 7Q1, 7Q2, 7Q5, 7Q20,
and 7Q25 lows as tabulated by Pyrce (2004) in Table 4.6. These 7Q X lows are also computed by
the DFLOW program discussed previously, using the methods described by USEPA (1986).

Uplands

Swamp

Marsh Lake

Infrequent high
Frequent high
Average
Frequent low
Infrequent low

Distance

FIGURE 4.28 MFL frequency. (From SJRWMD, Minimum lows and levels: Fact Sheet, St. Johns River
Water Management District, Palatka, FL, 2001.)