By Michael Howell
The data is in, according to Montana Bureau of Mines and Geology (MBMG) analyst Kirk Waren, from a three-year study of groundwater and surface water interaction in the shallow aquifer between Corvallis and Stevensville between the Union Ditch on the east and the main stem of the Bitterroot River on the west. The river bottom study area includes the Right Fork of the St. Mary’s Fork of the Bitterroot River, known locally as Mitchell Slough. The study is part of the MBMG’s Ground Water Investigation Program.
In 2009, the Montana State Legislature established the Ground Water Investigation Program (GWIP) within the MBMG to conduct detailed groundwater investigations in specific areas with the most serious concerns. Over forty projects were nominated and prioritized by the Groundwater Steering Committee. Seven investigations were initiated in 2009. Five additional areas were selected to begin during 2011 and 2012. Each investigation was expected to take from one to three years to complete.
A study in the Florence area was one of seven that were begun around the state in 2009. In 2012, studies were begun in the Hamilton area and in the Corvallis/Stevensville area in the river bottom.
GWIP Program Manager Ginette Abdo and Senior Research Hydrogeologist Kirk Waren gave the Ravalli County Land and Water Group an update on the Stevensville Shallow Aquifer study at the group’s regular meeting last week in Hamilton.
Abdo said that the MBMG was a non-regulatory agency that conducts unbiased research that is then provided to natural resource managers to help in their decision making processes. She said that analysis of the data in Florence was delayed until the full set of data from the Hamilton and Stevensville areas could be collected as well. All that data is now being analyzed and a detailed report describing the hydrogeologic system of the area and a comprehensive set of data will soon be made available to the public. She said the results are intended to provide a more detailed understanding of the groundwater system and tools which can then be used by regulators, senior water-right holders, new water-right applicants, and other stakeholders, to make informed water management decisions and to help anticipate hydrogeologic effects from changes in land use.
The focus of the study in the Hamilton area was based on increasing population density in the area and change in land use that may stress the aquifer and has raised the possibility of groundwater degradation.
The focus of the study along the river bottom was to gather information that might be helpful in determining the feasibility of using groundwater to supplement surface water for irrigation in the area.
According to Waren, it is meant to address concerns raised by irrigators about the expense and practicality of maintaining current diversion and distribution systems. One option the irrigators may consider in the future, he said, is moving some water diversions from surface water to groundwater wells. This investigation will provide them with the information needed to consider this option.
Six irrigation canals divert water from a reach of the Bitterroot River locally called the East Channel. Approximately 3,500 acres, including some of the most productive farmland in the Bitterroot Valley, are irrigated from this source. During the past several decades, irrigators have maintained flow into the Channel by excavating a canal from the main Bitterroot River to the East Channel. Construction and regular maintenance of the canal is necessary because the main river channel is migrating westward. The canal is presently about 3,000 feet in length and is being extended each year. There is significant concern among irrigators that maintenance of the canal will not technically or economically be feasible in the near future.
The study focused on the area lying between the Union Ditch and the main channel of the Bitterroot River beginning at the headgate and extending north (downstream) near the town of Stevensville. Two large irrigation systems, the Supply Ditch and the Bitterroot Irrigation District, are located above the Union Ditch. This project evaluated the scientific feasibility of using groundwater to supplement or replace irrigation water that is currently supplied by water diverted from the East Channel. Irrigation needs supplemented by groundwater may provide a more reliable source of irrigation water, particularly during droughts, and leave more surface water in the streams during periods when low flows are detrimental to fish and wildlife. A numerical groundwater model is being developed and used to evaluate various scenarios of groundwater use.
“This project is nearly ideal in terms of gaining a better understanding of groundwater-surface water interactions, a major goal of GWIP,” said Waren.
The products of this investigation will include an interpretive report and a groundwater flow model. These publicly available products will provide landowners and public agencies with scientific information to help make data‐driven water management decisions about how proposed changes in irrigation activities may affect groundwater and surface water in the area.