See Also
Our Changing Aquifer
Irrigation on the Eastern Snake River Plain Aquifer
Aquifer
Poster
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Aquifer basics: the bathtub concept
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| An aquifer can be thought of as a bathtub—a bathtub that, in the case of the Eastern Snake River Plain Aquifer, consists of thousands of cubic miles of porous, fractured basalt. Water from the faucet recharges the tub, water that goes down the drain (or is splashed on the floor) is discharged from the tub. Water in the tub is stored until the drain is opened or water is splashed out. When more water is recharged to the tub than drains, the water level in the tub increases and more water is in storage. |
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The Water Balance for an aquifer is:
Recharge – Discharge = Change in storage
We'll first talk about changes in storage because it helps us understand where the recharge to the aquifer stays before it can become discharge.
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| Aquifer Storage |
| The broken basalt and sediments of the Eastern Snake River Plain Aquifer contain a tremendous amount of water, as much as 1 billion acre-feet. This is enough water to cover the entire 10,800 square miles of the plain with nearly 145 feet of water, about the same amount of water as in Lake Erie. |
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Though the aquifer can be compared to the volume of Lake Erie, the aquifer is not at all like an underground lake. Water is stored between the grains of sediments and in the open fractures between pieces of basalt of the Eastern Snake River Plain. The water-holding rocks of the aquifer are as much as 4000 - 5000 feet thick. However, not all of that water can be easily used. Only 100 to 220 million acre-feet stored in the top few hundred feet of the aquifer can be easily pumped and used.
Flood irrigation practices (the only way to get waterto crops before sprinklers and electric pumps) add much more water to the crop than growing plants can use. The extra water soaks into the ground to add to storage in the aquifer, increasing the aquifer level beneath irrigated areas. An estimated 24 million acre-feet of water was added to the aquifer from 1880s to 1950s, with some places seeing water levels rise more than 100 feet.
But the longer we irrigated, the better we got at moving water to the places we wanted it. Irrigation methods changed from “flood” irrigation to more efficient sprinkler irrigation, and using only surface water to an increasing portion of pumped ground water. Increased pumping took more water out of the aquifer, and flood irrigation no longer provided as much recharge water. About 6 million acre-feet of water came out of storage from the 1950s to 1980, leading to aquifer level declines of fifteen feet or more in some areas. |
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From 1980 to 2002, another 6 million acre-feet of water has come out of storage in the aquifer, with aquifer-wide measurements showing an average 10 feet of decline since 1980, with half of that occurring in 2001-2002. Some areas, such as near Arco, have experienced even greater decline, more than 60 feet, while other areas have seen slight increases.
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Water level measurements from 2001 to 2002 have shown decreases in aquifer levels, largely due to the current drought.
Less snow in the mountains means less water in the river to irrigate with, less water to recharge the aquifer, and greater reliance on ground water. In many years the demand for surface water consumes most, if not all, of the flow of the Snake River above Shoshone Falls during the irrigation season. The result, first seen in 1905, is a dry "Twin Falls." |
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| Shoshone Falls: Nicknamed "the Niagra of the West," Shoshone Falls drops 50 feet farther than Niagra--212 feet. It is 1200 feet wide. |
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| Drawing from "Report of the Geological Exploration of the 40th Parallel," 1870-1880. Idaho State Historical Society. |
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| 1871. Settlers are arriving in Idaho, but few acres are irrigated. Surface water irrigation began to increase in the 1880s. |
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| Aquifer Recharge |
The water that fills the aquifer comes from a number of sources. The amount of water recharging the aquifer varies from year to year; however, the proportion of recharge from each source stays about the same. An estimated 8.06 million acre-feet of water recharged the Eastern Snake River Plain Aquifer in water year 1980. Because a great deal of measuring and sampling took place that year, it provides a good benchmark for comparison.
The largest source of water recharging the aquifer is irrigation. This leftover water seeps into the ground, and works its way to the aquifer. For the 1980 water year, this was 4.84 million acre-feet, or 60% of all recharge.
The next largest source of recharge to the aquifer is tributary basin underflow. That's ground water that flows to the aquifer from the tributary valleys along the margins of the plain. This includes recharge from the Henry's Fork and South Fork of the Snake River, and the valleys of Birch Creek, Big and Little Lost Rivers, Big and Little Wood Rivers, Portneuf and Raft River valleys, and other smaller valleys. This source added 1.44 million acre-feet, or 18% of recharge.
While the climate of the Eastern Snake River Plain Aquifer is semiarid, with less than 10 inches of precipitation each year, the timing of the rain and snow (snow cover melting and rain occurring in times of the year when there is less evaporation), and the scant soil cover over much of the basalts of the plain allows a significant amount of precipitation to recharge the aquifer in some areas. Direct precipitation on the plain accounts for 0.70 million acre-feet, or 9% of recharge.
Water infiltrating from the bed of the Snake River is also a significant source of recharge. Along some lengths ("reaches") of the Snake River, the riverbed is above the aquifer level; and therefore, water from the river seeps through the river bed to recharge the aquifer. These are called "losing reaches." Since aquifer levels can change during the year, some reaches of the river can "lose" during times of the year that the aquifer level is lower, and “gain” when the aquifer level is above the bed of the river.
In 1980, 0.69 million acre-feet, or 9% of recharge was from Snake River losses. Just like the Snake River , other rivers and streams, as well as canals, that flow out on to the Eastern Snake River Plain can recharge the aquifer. This recharge from tributary stream and canal losses added 0.39 million acre-feet, about 5% of the recharge for the 1980 water year. |
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| This postcard published in 1909 and these two pictures taken in 2005 show how the level of water going over Shoshone Falls rises and falls. |
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| Aquifer Discharge |
Just like the bathtub metaphor, what goes into an aquifer as recharge is reflected in changes in aquifer levels and in water discharged. Water can be discharged as springs in the walls of the Snake River Canyon , or seep into the bed of the Snake River in "gaining reaches," or be pumped out of the aquifer for use on the land.
In 1980, 8.22 million acre-feet were estimated to have been discharged from the aquifer. Most of this discharge, 7.1 million acre-feet or 86%, occurred as seepage and spring flow to the Snake River . Major springs occur along three stretches of the Snake River, near St. Anthony, from Blackfoot to American Falls, and Milner to King Hill. |
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Most of the spring flow and seepage occurs from Milner to King Hill, often called the Thousand Springs reach. Here, 4.83 million acre-feet or 68% of the spring flow and seepage occurs. Seepage and springs from Blackfoot to American Falls account for 1.99 million acre-feet, or 28% of discharge. The remaining 0.28 million acre-feet, or 4% occurs near St. Anthony.
Ground water pumped from the aquifer accounts for 1.14 million acre-feet, or 14% of discharge. Nearly all of this ground water is pumped for irrigation (95%), about 3% is pumped for drinking water for cities and rural homes. The remaining 2% is pumped for industrial and livestock use. |
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The pulse of the aquifer |
Spring discharge is like the pulse of the aquifer; changes in aquifer levels result in changes in spring flow. Measurements in some of the springs between Milner and King Hill began as early as 1902.
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As irrigation began in the Eastern Snake River Plain, spring flows from springs along the north side of the canyon increased. Estimates of spring flow (based on analyzing the water budget for years prior to 1951) were 4,200 cubic feet per second or about 3 million acre feet per year in 1902, and continued to grow until the early 1950s. At the peak flow in 1951, the discharge was estimated at 6820 cubic feet per second, and 4.94 million acre-feet.
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Between 1902 and the 1950s, irrigation with surface water spread across the Eastern Snake River Plain.
The increased recharge and aquifer levels resulted in a substantial increase in discharge from these springs. From the 1950s through 1980, the measured discharge from these ten springs decreased to about 6000 cubic feet per second, or 4.42 million acre-feet per year. Flow measurements made through 2002 show a continued decline to about 5400 cubic feet per second and 3.9 million acre-feet per year. The average spring flow from 1902 through 2002 is about 5800 cubic feet per second, or 4.2 million acre-feet of discharge. |
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The increase in spring discharge from 1902 through 1951 appears to be relatively constant, however, the decline from 1951 through 2002 is not. The fluctuations correspond to drought years that had less water available for surface water irrigation and wet years of higher flows in the Snake River , while the overall trend of decreasing flow from the springs is due to more acres being watered from sprinklers and less by traditional flood irrigation.
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Water users who designed their spring-dependent fish farms when flows were at their highest are now being affected by the decline in spring flows.
However, the decline in spring flows, outside of weather patterns that can't be changed, is due to the irrigators on the Eastern Snake River Plain becoming more efficient with their water in some portions of the aquifer, and in other areas by irrigators pumping ground water for their crops. |
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| An acre-foot is the amount of water which would cover one acre of land with water one foot deep. An acre is a little less than a football field from goal line to goal line; to be precise, from one goal line to within a football-length from the nine yard line at the other end of the field. This is 326,000 gallons. Discharge of one cubic foot per second is the same as 449 gallons per minute. One cubic foot per second would fill a football field one foot deep in 12 hours and 8 minutes. One cubic foot per second flowing from a spring for a year is 724 acre-feet. |
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