In the Black Hills the average annual precipitation is approximately 18 inches. This is more than the 1.92 inches received annually at Death Valley National Park but much less than the 133 inches received in a year in the Hoh Rainforest of Olympic National Park. Though the amount of rain that falls here is more than falls in the desert it is not sufficient to "water" the Black Hills. Groundwater is an important source of water for plants and animals here.
The high elevation of the area around Mount Rushmore means the watertable is charged by precipitation. The main water carrying rock formation is mica schist. The extensive tree and plant cover helps to control runoff, this encourages filtration of the water down into the broken and fractured rock. In spring, when the snow is melting, this ground cover is especially helpful. The ground cover will help to hold back runoff and offer moderate temperature control. If the snow melts slowly more of the water will filter into the rock and will not be lost as runoff.
To discover different ways nature helps to control runoff try this experiment at home or school. You will need two people, one to pour the water and another to run the timer or stop watch. Get a piece of plywood and cover it with a tarp. Prop the plywood up at a gentle angle and place a pan at the base of the plywood to catch the water. Using a 1-gallon watering can pour the water over the surface like rain. Time how long it takes the water to run down the board into the pan. Stop your timer when the water stops flowing. Now take a few soda cans and attach them to the tarp to simulate tree trunks and put torn pieces of wax paper on top to represent leaves on the tree. Scatter dirt, small twigs, leaves and pine needles over the surface of the tarp to represent forest debris. Again use your 1-gallon watering can to pour water over the surface, remember rain will fall on the treetops too. Time how long it takes the water to flow down slope. Which landscape did it take longer for the water to run through down into the pan? Which example would allow more water to soak into the ground? If you added more trees would that make a difference?
Around Mount Rushmore groundwater is used for drinking water and by the local plant and animal life. To provide drinking water for the people who live, work and visit the Mount Rushmore a well pumps water from the ground. The groundwater also comes to the surface on its own. Because of gravity water will flow downhill on the surface and in the bedrock. The water will continue to flow until it reaches the lowest point or it runs into something that will not let it pass. If there is a barrier the water will dam up. The water in the rock around Mount Rushmore flows downthrough the rock until it runs into granite or pegmatite dikes. The dikes act as dams and the water backs up behind them. The water can build up until it starts to flow at the surface forming a spring. Seeps and springs release groundwater unexpectedly from fractures in the rock as well. The seeps and springs provide water to creeks and small pools that might otherwise dry up. These are important watering places for the local wildlife population. The groundwater is also a water source for plant life. The shallow watertable of the area allows plant roots and especially the large taproot of trees to get water.
Around the Black Hills there is another interesting groundwater feature. Layers of metamorphic and sedimentary rocks ring the central granitic core of the Black Hills. Rings of sandstone and limestone in particular, are good rock layers for the formation of aquifers. The Madison or Pahasapa limestone and sandstones in the upper layer of the Minnelusa formation take in and hold groundwater. These rock layers are exposed at elevations higher than most of the state. The rest of the rock formation dips downslope. The down sloping will cause the water that is taken in to move downhill away from the intake area. The water level in the aquifer will rise as more water is added. The elevation to which the water will rise is called the potentiometric surface. This is a gently sloping line that moves downward and away from the intake area. Where the contour of the land surface goes below this line an artesian spring or well could form. Artesian springs are fairly common around the periphery of the Black Hills, especially on the west side in the Red Valley.
However, there are threats to the groundwater. Increased population causes strain as do resource development and periodic drought. The very thing that made the area popular with early settlers, mining the areas rich mineral resources, also endangers water quality. As population increases the need for additional water for drinking, household use, watering lawns, all take water from groundwater aquifers. Aquifers have a finite amount of water that they receive each year. Using more water than the aquifer is recharged with each year will cause the aquifer to be drawn down. Eventually the groundwater could someday be used up. To prevent this from happening there are things we all can do. Water your lawn in the evening or plant your lawn with native plants that are beautiful and drought resistant. Turn off the faucet while brushing your teeth, only use the dishwasher when it is full or stop using it all together. Get involved with your community, set an example for others to follow. Let your local, state and federal officials know that clean water is important for your community. Every little bit helps.