 |
The Importance of RiversRivers are invaluable to not only people, but to life everywhere. Not only are rivers a great place to play, but we use river water for drinking-water supplies and irrigation water, to produce electricity, to flush away wastes, to transport merchandise, and to obtain food. Rivers are indeed major aquatic landscapes for all kinds of plants and animals. Rivers even help keep the aquifers underground full of water by discharging water downward through their streambeds. And the oceans stay full of water because rivers and runoff continually refreshes them. Watersheds and RiversWhen looking at the location of rivers and also the amount of streamflow in rivers, the key concept to know about is the river's "watershed". What is a watershed? Easy, if you are standing on the ground right now, just look down. You're standing, like everybody else, in a watershed. A watershed is the area of land where all of the water that falls in it and drains off of it goes into the same place. Watersheds can be as small as a footprint in the mud or large enough to encompass all the land that drains water into the Mississippi River where it enters the Gulf of Mexico. Smaller watersheds are contained in bigger watersheds. The Blanco River watershed flows into the Guadulupe River watershed. It all depends of the outflow point — all of the land above that drains water that flows to the outflow point is the watershed for that outflow location. Watersheds are important because the streamflow and the water quality of a river are affected by things, human-induced or not, happening in the land area "above" (upstream of) the river-outflow point. Streamflow is Always ChangingStreamflow is always moving and adapting, from day to day and even minute to minute. Of course, the main influence on streamflow is precipitation runoff in the watershed. Rainfall causes rivers to rise, and a river can even rise if it only rains very far up in the watershed — remember that water that falls in a watershed will eventually drain by the outflow point. The size of a river is highly dependent on the size of its watershed. Large rivers have watersheds with lots of surface area; small rivers have smaller watersheds. Likewise, different size rivers react differently to storms and rainfall. Large rivers rise and fall slower and at a slower rate than small rivers. In a small watershed, a storm can cause 100 times as much water to flow by each minute as during baseflow periods, but the river will rise and fall possibly in a matter of minutes and hours. Large rivers may take days to rise and fall, and flooding can last for a number of days. After all, it can take days for all the water that fell hundreds of miles upstream to drain past an outflow point. The Blanco river isn't all that large, but since our storms tend to move from west to east, by the time we get rain in a big weather system, it may have already been raining at the Headwaters for two days. That can mean flooding, at least high water at the water crossings. 
|
