Surface water means any water that pools or flows on parts of the land’s surface, like streams and ponds. But a lot of water does not stay at the surface – it infiltrates, or seeps into the ground. Groundwater fills the tiny gaps in the soils and rocks below our feet (called pore spaces).
Groundwater & The Water Table
Before we can learn about groundwater-surface water interactions, we must understand the water table. The water table is the level at which the pore spaces are filled entirely with water. The water table is not a flat plain, but rather, a three-dimensional surface. Similar to the surface of the land, the water table has higher and lower areas. Parts of the water table are relatively steep while other parts are flatter. Like surface water, groundwater moves and flows, but much more slowly than surface water.
After a rainstorm or during snowmelt, water slowly seeps downward through the unsaturated parts of the soil to reach the water table. Water can also seep into and out of the ground through the bottoms of lakes and ponds and through streambeds!
Think About It…
Have you ever dug a hole in the sand next to an ocean or lake? Maybe if you dug deep enough, you noticed that your hole started filling in with water. This means you reached the water table! Water moves through the pore spaces between grains of sand to fill in the hole with water.
Gaining Streams
Streams are gaining when water is moving from the ground into the stream. If a stream is gaining, the amount of water flowing through the stream (the discharge) will increase as you move downstream.
Examine the stream diagram below. Arrows represent groundwater flow. What does the water table look like around the gaining stream?
Losing Streams
Streams are losing when water is moving from the stream into the ground. If a stream is losing, the amount of water flowing through the stream will decrease as you move downstream. In some cases, losing streams lose all their water into the ground and dry up at a certain point downstream.
Examine the stream diagram below. Arrows represent groundwater flow. What does the water table look like around the losing stream?
Other Possibilities
Additionally, some streams are gaining on one side and losing on the other. This is called throughflow. You can remember this because groundwater is basically flowing roughly perpendicular to the direction of streamflow, “through” the stream.
Examine the stream diagram below. Arrows represent groundwater flow. What does the water table look like around the stream with throughflow?
Furthermore, some streams may have gaining portions and losing portions. Streams or portions of streams may also be gaining at certain times of the year but losing at other times of the year, depending on different seasonal conditions. This is common in areas with seasonal monsoons or places with winter snowpack that melts in spring. Extended periods of drought can also alter the stream conditions.
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