Channel and Ground Water Relationships

Interactions between ground water and the channel vary throughout the watershed. In general, the connection is strongest in streams with gravel riverbeds in well-developed alluvial floodplains.

There are two types of water movement between streams and ground water:

• Influent or "losing" reaches lose stream water to the aquifer.
• Effluent or "gaining" reaches receive discharges from the aquifer.

Stream managers categorize streams based on the balance and timing of the stormflow and baseflow components. There are three main 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 the year. Seasonal flow in an intermittent stream usually lasts longer than 30 days per year.
• Perennial streams flow continuously during both wet and dry times. Baseflow is dependably generated from the movement of ground water into the channel.

Discharge Regime

Discharge is the term used to describe the volume of water moving down the channel per unit time. The basic unit of measurement used in 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 variables that determine the size and shape of the channel. There are three types of characteristic discharges:

• Channel-forming (or dominant) discharge. To envision the concept of channel-forming discharge, imagine placing a water hose discharging at constant rate in a freshly tilled garden. Eventually, a small channel will form and reach an equilibrium geometry. At a larger scale, consider a newly constructed floodwater-retarding reservoir that slowly releases stored floodwater at a constant flow rate. This flow becomes the new channel-forming discharge and will alter channel morphology until the channel reaches equilibrium.

  An estimate of channel-forming discharge for a particular stream reach can, with some qualifications, be related to depth, width, and shape of channel. Although channel-forming discharges are strictly applicable only to channels in equilibrium, the concept can be used to select appropriate channel geometry for restoring a disturbed reach. However, there is no method for directly calculating channel-forming discharge.

• Effective discharge. The effective discharge is the calculated measure of channel-forming discharge.

• Computation of effective discharge requires long-term water and sediment measurements, either for the stream in question or for one very similar. Since this type of data is often not available for stream restoration sites, modeled or computed data are sometimes substituted. Effective discharge can be computed for either stable or evolving channels.

• Bankfull discharge . This discharge occurs when water just begins to leave the channel and spread onto the floodplain. Bankfull discharge is equivalent to channel-forming (conceptual) and effective (calculated) discharge for alluvial streams at equilibrium.