2.5  Perennial, Intermittent and Ephemeral Streams

William W. Woessner

2.5 Perennial, Intermittent and Ephemeral Streams

In general, streamflow conditions can be described in terms of the presence/duration of flow, as continuous and discontinuous, and in most cases, are directly related to the nature of the groundwater exchange process. Streams referred to as perennial have flows year-round and are most often supported by base flow (effluent conditions) when runoff is insufficient to maintain discharge (Figure 22a). Intermittent streams flow only when sufficient groundwater discharge (effluent) and/or precipitation support flows. However, there are periods when the water table drops below the effluent portions of the channel and streamflow recharges the underlying groundwater. If channel leakage is high, all or portions of the channel become dry for a period of time. Ephemeral streams only flow in response to runoff as the water table generally remains below the channel bottom (Figure 22c). Most of the time influent conditions occur during channel flow. These streams remain dry when no runoff occurs.

Map view of stream channels illustrating groundwater exchange in three settings — **Figure 22 –** Map view of stream channels (long blue solid and dashed arrows; surface flow is from the upper left to the lower right) illustrating groundwater exchange in three settings. Small arrows indicate general exchange directions of groundwater (black arrows) and surface water (orange arrows). a) Perennial effluent stream where groundwater discharges to the stream and streamflow is maintained year-round. b) Intermittent streamflow is driven by precipitation and groundwater discharge. Sections of the stream may be gaining or losing during periods of full channel flow (left diagram). During a portion of the year groundwater inflows decrease and stream leakage (orange channel arrow) increases (right diagram of b) such that all or portions of the channel will become dry (red dot and dashed blue line). c) Ephemeral streams contain no streamflow until precipitation causes runoff. Streamflow seeps into the channel (influent). Channels become dry when runoff ceases and/or seepage rates exceed streamflow (Woessner, 2020).

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