Intermittent streams that regularly exist for only a short period of time are classified as ephemeral streams. Ephemeral streams are best illustrated by the dry stream beds in west Texas, called arroyos, that flow only following rainfall and cease to flow soon after.
Gaining and Losing Streams
What makes a stream perennial or intermittent? An important factor that allows some streams to maintain flow during dry weather is the water table. As rain falls or snow melts, some water percolates through the soil until it reaches an area where the pores and cracks in rock are saturated with water. This subsurface saturated zone is called an aquifer.
An aquifer stores groundwater. The upper area near the surface of the aquifer is called the water table. When the water table is at or very close to the surface of the earth, groundwater can be discharged into a stream as a spring. This type of stream is called a gaining stream (see Figure 4-5A).
When the water table falls below the stream channel, some water moves away from the surface to the aquifer. This type of stream is a losing stream (see Figure 4-5B). This concept is imporA