Channel and Ground Water Relationships
Interactions between ground water and the channel vary throughoutthe watershed. In general, the connection is strongest in streams withgravel riverbeds in well-developed alluvial floodplains.
There are two types of water movement between streams and groundwater:
- Influent or "losing" reaches lose stream water to theaquifer.
- Effluent or "gaining" reaches receive discharges fromthe aquifer.
Stream managers categorize streams based on the balance andtiming of the stormflow and baseflow components. There are threemain categories:
- Ephemeral streams flow only during or immediately after periods of precipitation. They generally flow less than 30 days per year.
- Intermittent streams flow only during certain times of theyear. Seasonal flow in an intermittent stream usually lastslonger than 30 days per year.
- Perennial streams flow continuously during both wet anddry times. Baseflow is dependably generated from themovement of ground water into the channel.
Discharge is the term used to describe the volume of water movingdown the channel per unit time. The basic unit of measurement usedin the United States to describe discharge is cubic feet per second(cfs).
Discharge is calculated as:
- Q = A V
where:
- Q = Discharge (cfs)
A = Area through which the water is flowing in square feet
V = Average velocity in the downstream direction in feet per second
As discussed earlier in this section, streamflow is one of the variablesthat determine the size and shape of the channel. There are threetypes of characteristic discharges:
- Channel-forming (or dominant) discharge. To envisionthe concept of channel-forming discharge, imagine placinga water hose discharging at constant rate in a freshly tilledgarden. Eventually, a small channel will form and reach anequilibrium geometry. At a larger scale, consider a newlyconstructed floodwater-retarding reservoir that slowlyreleases stored floodwater at a constant flow rate. Thisflow becomes the new channel-forming discharge and willalter channel morphology until the channel reachesequilibrium.
An estimate of channel-forming discharge for a particular streamreach can, with some qualifications, be related to depth, width,and shape of channel. Although channel-forming discharges arestrictly applicable only to channels in equilibrium, the conceptcan be used to select appropriate channel geometry for restoring adisturbed reach. However, there is no method for directlycalculating channel-forming discharge.
Effective discharge. The effective discharge is thecalculated measure of channel-forming discharge.
Computation of effective discharge requires long-termwater and sediment measurements, either for the stream inquestion or for one very similar. Since this type of data isoften not available for stream restoration sites, modeled orcomputed data are sometimes substituted. Effectivedischarge can be computed for either stable or evolvingchannels.
Bankfull discharge . Thisdischarge occurs when water just begins to leave thechannel and spread onto the floodplain. Bankfull dischargeis equivalent to channel-forming (conceptual) and effective(calculated) discharge for alluvial streams at equilibrium.
