Rivers and Streams, Characteristics - Hydro-Environmental Analysis: Freshwater Environments - page 43

FIGURE 2.27 (Continued)

be classiied as perennial or permanent, periodic , or episodic . Even in perennial rivers, lows
vary over time, such as within and between seasons. Peak lows and looding may occur, for
example, in the springtime due to runoff and snowmelt. Flooding is a major concern in river
management, as are low low conditions. As will be discussed in Chapter 3, management design
lows have historically been based on relatively rare high lows, for looding. For example, the
100-year low, or the low with a probability of occurrence during any given year of 1%, has
commonly been used to delineate the loodplain, with resulting regulatory implications. The
design of restoration projects is often also based on being able to withstand the 100-year low,
or some other relatively rare low event. The magnitude of the high-low event used for design
is usually based on the consequences of failure. For example, the design of dams in populated
areas is usually based on very extreme and rare events. Similarly, low lows for the management
of water quality or instream habitat have commonly been established based on relatively rare
low lows, such as the 7-day average low expected to occur once every 10 years (the 7Q10 low).
The determination of the magnitude of these relatively rare events, low or high low, is typically
based on a statistical analysis of historical data, as will be discussed in Chapter 3. However, as
will also be discussed in Chapter 3, it is the maintenance of the seasonal variations in the low
hydrograph that is becoming the more common goal of maintenance or river restoration efforts.
The maintenance of these seasonal changes may be necessary to maintain the characteristics of
a river system or to satisfy the seasonal variations in the needs and life stages of biota within or
contiguous to river systems.
Changes in the landscape and land use impact riverine characteristics over a variety of times-
cales. Changes in agricultural uses may vary over and between years, due to seasonal crop or pasture
demands, and crop or pasture rotations or forestry practices. Changes may also occur within and
between years as watersheds are developed, impacting iniltration and runoff. Large changes may
occur during and postconstruction. Stormwater regulations typically require that construction projects
reduce the peak runoff to that prior to construction. However, the timing and magnitude of runoff
may vary during and postconstruction. For example, as watersheds are developed, the percentage of