Montana Earth Science Picture of the Week

An "Artesian" Situation

Photo and diagram courtesy of Giant Springs Bottled Water Company

Pure and pleasant . . .
During the Lewis and Clark Expedition in 1805, Merriwether Lewis wrote, “I think this fountain the largest I ever beheld, extremely transparent and cold, very pure and pleasant”. Lewis was referring to a place called Giant Springs where water comes out of the ground along the Missouri River on the northern edge of Great Falls. Nearly 200 million gallons of water per day come out of the springs and then flow into the Missouri River some 201 feet away. Called the Roe River, this flow is arguably the shortest river in the USA.

Three Requirements . . .
The diagram at the top of this page shows the artesian nature of the aquifer that brings the water to Giant Springs. The word “artesian” indicates that the water is forced to the surface by hydrostatic pressure (without pumping). For this to happen, three conditions must be present. For one, the aquifer must be sandwiched between impermeable layers (layers that won't let water soak in). In the case of Giant Springs, the aquifer is a layer called the Madison Limestone. It lets water enter and pass through it, whereas the layers of rock above and below it do not. A second requirement is that the aquifer must be sloped as the Madison Limestone is between the Little Belt Mountains and Great Falls. Finally, the spring (or well) must be at an elevation that is lower than the elevation of the recharge area. This is also true with Giant Springs. Its waters originate as precipitation that falls on the Little Belt Mountains over 4,000 feet above the Missouri River. At Giant Springs pressure forces water up from a depth of 700 feet through cracks in the rock layers above the limestone. The water emerges at Giant Springs, some 40 miles away from the recharge area in the Little Belt Mountains where it fell as precipitation.

Several years ago, scientists used radiocarbon dating to determine the age of trace amounts of carbon contained in the water to determine that the water takes 2,900 years to travel from the mountains to the river. The results were later found to be invalid.

NOTE: Giant Springs State Park is a short walk from the Lewis and Clark Interpretive Center.

Terms: recharge area, impermeable


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By Rod Benson
Earth Science Teacher at Helena High School

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