Georgia NWI Help

STREAMFLOW MAINTENANCE

There are four main sources of water to support stream flow: 1) groundwater, 2) interflow through the soil, 3) precipitation, and 4) surface water runoff. Groundwater provides water for base flows. Many wetlands are sources of groundwater discharge and those located in headwater positions either the source of streams or along low-order perennial streams contribute significantly to sustain streamflow in the watershed. Such wetlands are critically important for supporting aquatic life in streams. The importance of maintaining natural streamflow patterns is important to riparian vegetation as well as to resident aquatic species and altering those patterns can negatively impact local biodiversity (Cowell and Stoudt 2002).

All wetlands classified as headwater wetlands are important for streamflow. Terrene headwater wetlands, by definition, are sources of streams. They contribute groundwater (base flow) from local unconfined aquifers and regional confined aquifers to support streamflow (Priest 2004). Other headwater wetlands include lotic wetlands along 1st-order streams and lentic wetlands associated with outflow lakes. Wetlands along 2nd-order streams in mountainous areas may be classified as headwater wetlands as they probably are sites of groundwater discharge, but these conditions do not apply to the Georgia coastal region since Georgia’s mountains are much further inland. Ditched headwater wetlands are rated as moderate, since this alteration typically results in faster release of water, thereby reducing the period of outflow. Outflow from groundwater-fed wetlands (lacking a stream) may discharge directly into streams and thereby contribute variable quantities of water for sustaining baseflows. These wetlands were rated as moderate for this function. Lakes may also be important regulators of streamflow, so lentic wetlands may be designated as significant to streamflow, with those in headwater positions being rated high and others as moderate.

Floodplain wetlands are known to store water in the form of bank storage, later releasing this water to maintain baseflows (Whiting 1998). Among several key factors affecting bank storage are porosity and permeability of the bank material, the width of the floodplain, and the hydraulic gradient (steepness of the water table). It is recognized that the wider the floodplain, the more bank storage given the same soils. Gravel floodplains drain in days, sandy floodplains in a few weeks to a few years, silty floodplains in years, and clayey floodplains in decades. In good water years, wide sandy floodplains may help maintain baseflows. Bank stratigraphy is another factor that could be considered important for streamflow maintenance (Christopher Cirmo, pers. comm. 2006). For example, the presence of a “sand” layer between clay layers (such as in a system where there have been historical floods) may affect the transmissivity of the bank. Bank storage may serve to maintain streamflow in some fringe or floodplain wetlands, however a rudimentary knowledge of the surficial stratigraphy is not normally available based solely on remote data interpretation. Despite the variability in floodplain properties, the W-PAWF assessment treats all nontidal floodplain wetlands and stream basins as having potential to support streamflow, since remote sensing data does not include soil examinations or bank stratigraphy and there is no recognized floodplain width designated to separate high from moderate potential.

While diked ponds may contribute to streamflow when water overflows spillways or exceeds height of water-control structures, these ponds typically reduce streamflow (McMurray 2007, Van Liew 2004). However, some ponds may extend storm-flow over longer durations by reducing peak flows (Bosch et al. 2003). Since impounded ponds are artificially created waters that substantially alter natural streamflow characteristics they are not included as significant for streamflow maintenance.

For this function, the following relationships are used:

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