Legal Definitions - underflow of surface stream

The term underflow of surface stream refers to the movement of water beneath the visible surface of a river or stream. This subsurface flow occurs through the permeable soil, sand, gravel, or other porous material that makes up the streambed and banks. It is hydraulically connected to the main body of water in the stream, meaning it moves in the same general direction and is influenced by the stream's water level, even though it is not visible on the surface.

Here are some examples to illustrate this concept:

• Example 1: Desert Riverbed

  Imagine a river flowing through a desert landscape. During the dry season, the visible water on the surface might shrink to a narrow channel or even disappear entirely in certain sections, leaving a seemingly dry riverbed. However, if you were to dig a shallow hole in that "dry" riverbed, you might quickly find water seeping in. This hidden movement of water through the sand and gravel beneath the visible surface is the underflow of the surface stream. It's still part of the river's system, just flowing out of sight.

• Example 2: Well Near a Creek

  A homeowner has a shallow well dug on their property, located only a short distance from a small, meandering creek. During a prolonged drought, the creek's visible flow becomes very low, almost stagnant. Despite this, the homeowner's well continues to provide a steady supply of water. This is likely due to the underflow of the surface stream. Even with minimal surface water, the creek's water is still moving slowly through the porous soil and gravel beneath its bed and banks, replenishing the groundwater that the well taps into.

• Example 3: Environmental Water Management

  A state water resource agency is assessing the total water available in a particular river basin to allocate water rights for agriculture and municipal use. Beyond measuring the visible flow of the main river, they also conduct geological surveys and install monitoring wells along the river's course to estimate the volume of water moving through the alluvial deposits (sand, silt, and clay laid down by the river) beneath the riverbed. This additional volume of water, which is not visible but is hydraulically connected to the river, represents the underflow of the surface stream. Understanding this underflow is crucial for accurately managing the basin's overall water budget and ensuring sustainable use.