Rivers and Streams, Characteristics - Hydro-Environmental Analysis: Freshwater Environments - page 30

deines a perennial stream as “a well-deined channel that contains water year round during a year
of normal rainfall with the aquatic bed located below the water table for most of the year” and
“biological, hydrological, and physical characteristics commonly associated with the continuous
conveyance of water.”
Whether a stream or a river is perennial, intermittent, or ephemeral is most commonly a func-
tion of the relationship of the water level to that of the groundwater (Figure 2.17), and whether the
stream is a losing or a gaining system. A gaining stream (efluent) is below the water table and
receives discharges from the aquifer. An inluent or “losing” reach loses stream water to the aquifer.
An ephemeral stream is a losing stream and lows only during or immediately after periods of pre-
cipitation. Intermittent streams are also averaged over time as losing streams and low only happens
during certain times of the year. During periods of dry weather, they may dry up but leave a series
of disconnected pools that can sustain aquatic life. Perennial streams are gaining systems that low
continuously during both wet and dry periods. Perennial streams can also result from lows from
origins other than groundwater. For example, in arid regions, some perennial streams exist due to
industrial or wastewater discharges.
The low in streams may be considered to consist of two components, the base low (for perennial
streams) and the storm low as illustrated in a graph of low versus time (Figure 2.18). The storm
low results primarily from runoff from the watershed and the peak low occurs some time after the
peak rainfall (a lag time). The hydrograph shows a rising and falling limb as runoff begins and ends.
The low in perennial rivers not only occurs when it rains, but also because there is a base low
component that is primarily a function of groundwater inlow from shallow aquifers or discharges
from industries or municipal wastewater discharges. As a result, the quantity (and often the quality)
of rivers during periods between storms is integrally linked with the groundwater table. The ground-
water table can vary as a function of the recharge (from the iniltration of rain) and withdrawals,
such as from wells. Groundwater withdrawals for agriculture or other uses have resulted in lowering
the water table in many areas, such as the Mississippi Delta, with the consequent impact that the
average low (base low) in many streams has declined over the years and many formerly perennial
streams are now ephemeral or intermittent. The interrelationship between groundwater withdrawals
and surface water supplies is critical to many water use and water management programs.

2.2.2.1.2 Hyporheic Zone
A rheic low refers to the visible free-running water we normally think of as a stream, river, or other
moving low of water, while hypo refers to “under, beneath, or below.” So, the hyporheic zone is

Water table

Water table

(a) Influent stream reach (b) Euent stream reach

FIGURE 2.17 (a,b) Groundwater and surface water interactions. (From FISRWG, Stream Corridor
Restoration: Principles, Processes, and Practices , Federal Interagency Stream Restoration Working Group,
1998.)