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Water Flows Underground from Little Belt Mountains

in Central Montana to Manitoba

by David Baker

 

Figure 1. Cliff of Madison Limestone in Sluice Boxes State Park in late summer. Most of Belt Creek disappears underground into the Madison Limestone Aquifer.

Here in the Little Belt Mountains in Central Montana we have creeks in which a portion goes dry for a while in the summer and in the winter. One example is Dry Fork (of Belt Creek), which joins Belt Creek at Monarch. In dry summers and in periods of low flow in the winter Dry Fork is empty for several miles upstream from Monarch. In very dry summers several miles below Monarch all of Belt Creek disappears into the ground and there is a dry stretch of empty creek bed; however, both upstream and downstream the creek has water. Every summer and winter there are times when Belt Creek dries up for several miles above Armington Junction. The U.S. Geological Survey measured stream flow (or discharge) in creeks in the Little Belt Mountains and found that the creeks lost significant amounts of water where they crossed carbonate rocks--limestone or dolomite. The largest loss occurs in the 1700 foot thick Madison limestone. Monarch Canyon and the Sluice Boxes are carved in Madison limestone. As shown in Figure 1, water disappears into this limestone and flows underground through a connected network of cracks, fissures, crawlways, tunnels, and caves.

The Little Belt Mountains are formed as a large fold in the layered rocks--about 30 miles wide and 80 miles long (Figure 2). The axis goes through Neihart. Beds north of Neihart are tilted north towards Great Falls. As shown in the cross section the groundwater flows downhill in the Madison limestone aquifer towards Great Falls where there is a leak in the system, known as Giant Springs.

Figure 2.  Cross section from the Kings Hill Area of the Little Belt Mountains to Giant Springs at Great Falls. The thick layer is the 1700 foot thick Madison Limestone.  Note the fold structure of the Little Belt Mountains.  Dashed lines show where the limestone was before it was removed by erosion.  Water in Belt Creek enters the limestone between Monarch and the lower parking lot for Sluice Boxes State Park and comes out at Giant Springs.  Vertical exaggeration = 10X.  8000 and 2000 are elevations above sea level.

At Giant Springs the Madison limestone is less than 400 feet below the surface and groundwater from the Little Belts escapes upwards through cracks in the overlying Kootenai sandstone (Figure 3).  (These cracks are easily seen from the concrete walkway over the springs.)

Figure 3.  At Giant Springs groundwater flows out of vertical cracks (known as joints) in Kootenai Sandstone.

Although much water flows out at Giant Springs, a significant leak in the Madison limestone aquifer, water in this aquifer flows eastward from the Little Belt Mountains across Eastern Montana towards the Williston Basin (Figure 4). The groundwater is deflected around the north side of the Williston Basin and flows into Canada. Groundwater from the Little Belt Mountains finally comes to the surface in a series of saline seeps along the shores of Lake Winnipegosis and Lake Manitoba in the Province of Manitoba after traveling 1000 kilometers or 600 miles underground.

Figure 4.  Flow of groundwater in the Madison Limestone Aquifer, the largest artesian aquifer in the United States.  The Little Belt Mountains are one of the main recharge areas for the aquifer.  Others include the Big Snowy Mountains, the Big Horn Mountains, and Black Hills of SD.  Discharge area is in Manitoba.  Giant Springs is a "leak" in the aquifer.

In Central Montana groundwater in the Madison limestone contains mostly calcium (and magnesium) carbonate which was dissolved from the limestone. However, in the Williston Basin and in Canada the groundwater in the Madison aquifer becomes a saturated brine solution containing mostly sodium chloride.

It takes several thousands of years for groundwater to reach Giant Springs, but tens of thousands of years to travel underground to Manitoba.

Questions and Answers

Links to Other Sites for Those Who Want to Know More

Neihart is a (lead, zinc, silver) mining town in the center of the Little Belt Mountains. The treeless, snow-covered upper slopes in the photo above (Neihart Baldy) are 1.5 billion year old metamorphosed sandstone, called the Neihart quartzite. The tree-covered slopes below are basement rock which was highly metamorphosed 1.8 billion years ago, but has some portions that are 2.7 billion years old. As indicated in Figure 1, the Madison limestone has long since been eroded off the top of the Little Belts. The peak adjacent to Kings Hill Pass is Porphyry Peak, a small 50 million year old igneous intrusion, which is the site of the Showdown Ski Area. Click here for a local area map showing location of Monarch, Neihart, Belt, Armington,, and White Sulphur Springs. Armington Junction is shown as the intersection of U.S. 89 and MT 200. Dry Fork is a tributary that enters Belt Creek at Monarch.

Cliff of Madison limestone rising above Belt Creek. A short distance downstream from this spot water disappears into the limestone aquifer. In August in very dry years when the water is low, all of the flow of Belt Creek goes underground there.

Sluice Boxes--the location of a "slot" cut into Madison limestone by Belt Creek. The "slot" is about a hundred feet deep and 30 feet wide and is about a mile above the railroad trestle shown in the photo. Sluice Boxes State Park.

Giant Springs is located next to the Missouri River, east of Great Falls. It is a "leak" or discharge point for the Madison Limestone aquifer. The site is now a State Park and the source for bottled water.

Karst features, such as tunnels, caves, and sinkholes, form in limestone as it is being dissolved.  The name, karst, comes from a limestone region in Slovenia, part of the former Yugoslavia, where the caves and cave-formations are world-famous. One of my field trips includes a tour of Lick Creek Cave (formed in Madison limestone) in the Little Belt Mountains. It has a room 500 feet in diameter with an 80 foot ceiling. Noteworthy caverns developed in Madison limestone include the Lewis & Clark Caverns near Three Forks and Wind Cave and Jewel Cave in the Black Hills of South Dakota. To see a good photo of a tunnel in Madison limestone, click on the Groundwater Atlas of the U.S., then on "Principal Aquifers", then scroll down in the text to "(Fig. 39)".

The Williston Basin is a roughly circular-shaped area in western North Dakota, eastern Montana and southern Saskatchewan that has been subsiding very slowly since the Cambrian. The bowl shape of this basin is apparent in an isopach map, showing the thickness (in meters) of Devonian age sediments. To access a professional-level description of the Williston Basin with many illustrations go to Alberta Geological Survey website, click on "Atlas of the Western Canada Sedimentary Basin" and scroll to Chapter 27.

A cross section through the Williston Basin in the Groundwater Atlas of the U.S., not only shows that the basin has subsided, but also shows that the beds are thicker in the basin than elsewhere.  They record the half billion year history of subsidence. To view cross section A-A', click on the Groundwater Atlas of the U.S., then on "Regional Summary", then scroll down in the text to "(Fig. 11)". The location of the cross section is shown on the geologic map in (Fig. 10).

Deflection of groundwater. Groundwater in Madison limestone in the Williston Basin is a completely saturated brine--a saltwater solution that contains so much sodium chloride that no more salt will dissolve in it.. The salt comes from thick salt beds (evaporites) at the top of the Madison limestone. The brine has a higher density than fresh water coming from Central Montana. The fresh water stays high and flows around the Williston Basin rather than going down through the Basin. Water coming from the Little Belt Mountains flows around the north side of the Williston Basin, whereas water coming from recharge areas in the Bighorn Mountains and the Black Hills of South Dakota flows around the south side of the Williston Basin.

Saline seeps or saltwater springs are the places in Manitoba where groundwater that has traveled through the Madison limestone aquifer flows out on the surface. The underground journey around the northern side of the Williston Basin, where there are extensive salt beds, changed the character of the groundwater so that it is now saltwater or brine. Click here for a photo of the Lake Winnipegosis Salt Flats (on the second page of the brochure). Charles Burchill studied the vegetation in these springs for his M.Sc. Thesis.

Questions and Answers

Introductory Level Q & A (suitable for a 9^th grade earth science class):

1. Question: From Central Montana to Central Manitoba is 1000 kilometers or 600 miles. That is a long ways for water to travel underground. What makes it flow so far? Answer: Gravity. Water flows downhill. The elevation of Belt Creek below Monarch where water goes into the Madison limestone is about 4500 feet (1370 meters) above sea level. The elevation at Giant Springs is 3230 feet (985 meters) and at Lake Winnepegosis, 830 feet (253 meters). On the topographic map of Montana note how the Little Belt Mountains in the center of the state are high and the northeast corner of Montana is low. If you pour water into the high end of a long garden hose, the water will run out the low end, provided that the hose is not plugged. The Madison limestone aquifer has the geometry of a porous sheet rather than an open tube.

2. Question: Why doesn't the water leak out to the surface? What keeps it underground for so long and for such a great distance? Answer: There are layers of shale above and below the Madison limestone which keep the water confined so that it has to stay in the limestone. Shale, which was deposited originally as mud, makes a good barrier for underground water. Water can not flow through shale very easily, whereas the limestone contains interconnected holes in a large range of sizes. Water flows easily through the limestone. To see a diagram showing the Madison limestone with the confining layers of shale, click on the Groundwater Atlas of the U.S., then on "Regional Summary", then scroll down in the text to "(Fig. 14)". The Madison aquifer is the one in the middle.

3. Question: Limestone is supposed to be a solid rock. How come it has so many holes? Answer: The limestone was originally deposited when sea shells or skeletal remains fell on the floor of a shallow sea. Most of the animals were one-celled animals living near the surface of the sea. When they died the hard parts, made of calcium carbonate or calcite, "rained" on the seafloor, like a continual snowfall. The space between the shells, which is known as the pore space, contained seawater. However, during compaction and diagenesis most of the pore space was filled by calcite. Eventually the seafloor was uplifted, and the shoreline retreated. Vertical fractures, called joints, formed in the limestone during uplift. Fresh water started to flow very slowly through the fractures in the limestone. The fresh water contained dissolved carbon dioxide, which made it slightly acidic. This weak acidity slowly dissolved some of the limestone. The longer that groundwater flowed through the limestone, the more limestone was dissolved. A network of passages that follow the systematic joints is a common configuration in limestone caves. The map of over 100 miles of passages in Jewel Cave shows the pattern of several sets of intersecting systematic joints.

4. Question: How do geologists know which way water flows underground?

Answer: A special map called the potentiometric surface is the main tool for determining the pattern of underground flow. It documents how water flows downhill, even when it is underground. Many water wells and oil wells have been drilled into the Madison limestone. It is easy to measure how deep the water is in each well when there is no pumping going on or the height that water would rise if a pipe were attached above ground. (In the latter case one measures the water pressure and converts the value to the height of a column of water that would generate that pressure.) By plotting this data (expressed as elevations above sea level) on a map and contouring, one can generate a potentiometric surface for the aquifer (Φ₁, Φ₂, Φ₃, Φ₄).  The map looks like a topographic map. Groundwater flows in the downhill direction, perpendicular to the contour lines.  The flow lines are labeled Ψ₁, Ψ₂, Ψ₃, Ψ₄ .

In the map on the right the blue areas are mountains where the Madison Limestone is exposed on the surface and where there is mountainside above the limestone.  Most of the surface runoff in the blue area goes into the Madison Limestone Aquifer.  The blue areas are major recharge areas for the aquifer.  The contours show the potentiometric surface for the Madison Limestone Aquifer out on the prairie away from the mountains.   The potentiometric surface follows the topography in central Montana but is more subdued. In the Little Belt Mountains, where there is Madison Limestone, the potentiometric surface is below the ground and surface water disappears into the ground.   Out on the prairie the potentiometric surface is above ground because this is a confined aquifer.  Water wells are artesian wells with water flowing out of the well at the surface.  The closed contours, shown in red, surround the discharge point at Giant Springs.

The flow lines (shown as heavy lines with arrow heads) go to Giant Springs, showing that this is a very large "leak" in the aquifer.

5. Question: How do geologists know how long it takes for water to travel underground? Answer: Radiocarbon dating is used to determine how long the groundwater has been underground. Cosmic rays bombard the atmosphere converting some nitrogen into radioactive carbon. In particular, when a slow neutron hits a nitrogen atom, ₇N¹⁴, the neutron replaces a proton to produce the carbon isotope, ₆C¹⁴. This isotope has a half life of about 5730 years. The carbon reacts with oxygen in the atmosphere to make carbon dioxide. Carbon dioxide dissolves in rain water and surface water. The ratio of radioactive ₆C¹⁴ to ordinary carbon ₆C¹² remains about the same as long as the water is in contact with the atmosphere. However, as soon as water goes underground the carbon 14 concentration decreases. By measuring the carbon 14 concentration (and correcting for the non-radiogenic carbon in carbonate that came from dissolved limestone and other effects), the age of the groundwater can be determined. Groundwater at Giant Springs has been dated as older than 2900 years, but younger than 6300 years. The flow rate is roughly 50 feet per year from the Little Belt Mountains to Giant Springs compared with about 1 to 2 feet per year for the trip across the prairie to Manitoba.

6. Question: Why are the Little Belt Mountains the recharge area for the Madison limestone aquifer? Answer: To see the maps and diagrams in this section click on the Groundwater Atlas of the U.S., then on "Regional Summary", then scroll down in the text to the relevant figure "(Fig. ??)" and click. East of the Rocky Mountains there are a series of dome-shaped uplifts (Fig. 12), separated by down-dropped basins. These basement-cored uplifts are known as the foreland ranges or the Laramide ranges. Erosion has removed the top of the domes, revealing basement rocks in the core and the sequence of sedimentary formations on the flanks, including Madison limestone (Fig. 14). Surface water enters the Madison limestone where the limestone is exposed on the surface. Recharge areas for Madison limestone are labeled on the map in (Fig. 7) as "Paleozoic aquifers". In central Montana these are the Little Belt and Big Snowy Mountains with minor contributions from the Little Rocky, Moccasin, and Judith Mountains.  The other major recharge areas are the Bighorn Mountains and the Black Hills of South Dakota.

7. Question: How can groundwater flowing underground cross the continental divide? Answer: The Missouri River does indeed carry surface water to the Gulf of Mexico. However, groundwater that has traveled through the Madison limestone aquifer around the north side of the Williston Basin is discharged in saline seeps that flow into Lake Winnipegosis or Lake Manitoba and then into Hudson Bay. The groundwater flow, which normally follows the surface flow pattern, is deflected by the dense brines in the Williston Basin. The Missouri River flows across the Williston Basin without hindrence, but groundwater in the Madison limestone aquifer is mostly deflected around the basin.

9. Question: How have plate tectonic motions determined the flow of groundwater in the Madison aquifer? Answer: [UNDER CONSTRUCTION]

10. Question: Discuss the hydrologic model for the Madison limestone aquifer and its numerical simulation. Answer: [UNDER CONSTRUCTON]

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