Updated: Jan 12, 2022
A few months ago I was contacted by Michael Downey, a water resources planner in Montana's Department of Natural Resources Conservation (DNRC). He had read about a study I directed a few years ago that investigated the effects of development on hydrology and water management in the Henry's Fork watershed and invited me to come talk about this study--and more generally about groundwater management in Idaho--at a day-long technical workshop in Helena. I just returned from that workshop and thought it relevant to share both my presentation and a few interesting observations with you.
The technical workshop was attended by about 40 people from around Montana, mostly scientists and managers from DNRC and other state agencies. Other participants included stream restoration consultants, university faculty, graduate students, and staff of nongovernmental organizations.
First, the presentation. The slides are linked here and are pretty self-explanatory. The most important points of my presentation were:
A large amount of water is withdrawn from streams for irrigation in the Henry's Fork watershed, but only about 25% of the water withdrawn is actually consumed by crops. About two-thirds of the water withdrawn seeps from canals and fields and recharges local aquifers, and nearly half of that returns to the river within the watershed. The rest flows out of the watershed as groundwater, contributing to the Eastern Snake Plain Aquifer, which eventually discharges the water back to the Snake River farther downstream.
Conversion of irrigation from flood application to sprinkler application, as well as conversion of some canal systems to pipelines, has reduced groundwater recharge and increased consumptive use of water over the past 30 years. This has reduced streamflows in the lower Henry's Fork and lower Teton River during late summer when streamflows are already at their lowest.
Irrigation seepage is the largest source of recharge to aquifers throughout eastern Idaho.
Only about 5% of irrigated agricultural land in the Henry's Fork watershed has been developed for residential use, but almost all new development in the watershed has occurred on land formerly irrigated by surface water delivered in those leaky canals.
When land is removed from agricultural production a variety of sociological, legal, and physical factors result in increased use of groundwater and decreased groundwater recharge--a double whammy on our groundwater supplies and associated ecological benefits.
State of Idaho water policy promotes recovery of the state's aquifers through a number of programs, including maintenance of existing recharge associated with canals and flood irrigation, reduction in groundwater-irrigated acreage, use of canal-delivered surface water to irrigate landscaping in new developments, and managed recharge of groundwater.
My presentation followed an excellent presentation from John LaFave and Ginette Abdo of the Montana Bureau of Mines and Geology, who showed figures on canal seepage from several irrigated regions of Montana. Their quantitative results were nearly identical to those I and my graduate students have calculated for the Henrys Fork watershed and in fact for the whole upper Snake River system. Together, our two presentations indicated to the group that maintaining existing canal-served irrigation, particularly when applied using flooding or other surface methods, is important to maintaining existing groundwater recharge.
However, most of the other presentations focused on floodplain management and restoration as a method to increase groundwater recharge and its ecological benefits. The reason for this focus is that unlike in Idaho, where we have extensive aquifers in highly permeable volcanic rocks, most of the high-quality groundwater in Montana occurs in relatively small, shallow aquifers in river floodplains. Under natural hydrological and geological conditions, annual flooding during the snowmelt period inundates these floodplains with water, some of which seeps into the permeable soils and gravels of the floodplains. After streamflows recede, this stored groundwater gradually flows back into the river--just like irrigation seepage--maintaining higher streamflows later into the summer and fall.
However, these natural floodplain processes have been disrupted in many of Montana's rivers (and Idaho's, for that matter) by a variety of activities including channelization, construction of roads and railroads in floodplains, and flow alterations by dams. Restoration of these processes is an important component of Montana's new state water plan, because it provides a natural way to store water without building new storage facilities. In addition, restoration of floodplains restores important habitat for a variety of fish and wildlife species. Fish receive the double benefits of increased side-channel habitat and increased late-summer streamflows from such restoration.
The tricky part is that while everyone in the room supported restoration of floodplains for the multiple resource benefits, Montana law is not completely clear on what restoration techniques are allowable without jeopardizing existing water rights. For example, one restoration technique that has been successfully applied to small streams is construction of "beaver-dam analogues," essentially small porous dams made of sticks. These small dams mimic beaver dams. But, while beavers do not need a water right to construct their dams, humans might, even if the dam mimics the work of beavers and is intended to restore ecological processes.
Furthermore, Montana has not formally established water rights and accounting procedures for using water for managed aquifer recharge. In that sense, Idaho's water law, policies, and administrative processes are decades ahead of Montana's. I heard that comment several times throughout the day, as we compared and contrasted Idaho's and Montana's geology, hydrology, and water management systems.
Regardless of differences between the states, one thing was clear. Groundwater resources in both states are at risk from a variety of factors, and reduction in groundwater levels is bad news for late-summer and fall streamflows, which are going to become lower and warmer under projected future climates. Enhancing groundwater storage--whether through managed aquifer recharge, return to more use of flood irrigation, or restoration of natural floodplains--is important to stabilizing water supplies and providing more water for fish in both states.
Through its ongoing water-quality and water-quantity research, participation in the Henry's Fork Watershed Council, and involvement in regional water management initiatives, the Henry's Fork Foundation is furthering both the science and practice of groundwater management. The opportunity to share knowledge with our colleagues in Montana helps us contribute to improved trout fisheries in both states.