Overview/Preface

The Clark Canyon Reservoir on the Beaverhead River, south of Dillon, MT, serves as a water storage and supply facility for two irrigation projects including the East Bench Irrigation District and the Clark Canyon Water Supply Company. The East Bench Irrigation District consists of 21,800 authorized acres and the Clark Canyon Water Supply Company consists of 28,000 authorized acres. Off-season and spring snowmelt water is stored within Clark Canyon Reservoir and released for irrigation diversion during the crop growing season.

Water supplies to irrigators within the East Bench Irrigation District are directed through one single diversion, located at Barrett’s Diversion Dam. The Clark Canyon Water Supply Company consists of many diversion points from the river, many of which serve just a few individuals. Most of these diversions are located down-stream of Barrett’s Diversion Dam and the point of diversion to East Bench Irrigation District.

Jim Bauder and Kim Hershberger install an AquaRod at the Red Rock monitoring site, east of the Clark Canyon Reservoir.

The area encompassing these two irrigation projects is essentially 1) the irrigated acreage within the alluvial river channel of the Beaverhead River from Barrett’s Diversion Dam to the confluence of the Beaverhead River with the Ruby River at Twin Bridges, and 2) portions of the alluvial “East Bench” within the same area. The land area encompassing the two irrigation projects and irrigated acreage above Barrett’s Diversion Dam consists of a corridor of approximately 4 miles x 50 miles in size, dissected by U.S. highway 91 and State highway 41.

The water service contract between the Bureau of Reclamation and both the entities (East Bench and Clark Canyon Water Users) is set to expire on December 31, 2005 and is being reviewed for renewal. The current contract provides 4 acre-feet of water per acre to the shareholders of the Clark Canyon Water Supply Company as measured at the river headgates as first priority (112,000 acre-feet total) and 3.1 acre-feet per acre for each of the irrigable acres for the East Bench Irrigation District measured at Barrett’s Diversion Dam as second priority (65,580 acre-feet total). Collectively, diversions may amount to as much as 179,580 acre-feet when available.

Project Goal

Of primary interest in this study is the distribution of water among the various entities withdrawing water from the Beaverhead River. Water measurement at the point of diversion from the river is necessary to effectively distribute water amongst water users along the river in accordance with priority dates and allocations. Currently, the East Bench Irrigation District measures discharge into their canal at Barrett’s Diversion Dam as required by the current water supply contract. In contrast, suitable water measurement devices are lacking at a majority of river diversion points for the Clark Canyon Water Supply Company. Some diversions have water measuring devices, but they are either in-appropriate for the location or were not properly maintained and are in need of replacement. The lack of water measurement leads to both inequality of water distribution and makes management of the river and the storage reservoir challenging.

In order to address the issues of water allocation, distribution, and lack of quantifiable flow data, the primary goals of the project reported here were to develop a monitoring/recording network, gather data, develop an inventory, and complete a water budget for the 2004 irrigation season. This water budget involved:

1) Identifying major diversions from the river associated with the Clark Canyon Water Supply Company between Clark Canyon Reservoir and Twin Bridges, MT

2) Identifying major tributaries and return flows associated with the Clark Canyon Water Supply Company including tributaries to Clark Canyon Reservoir, and all tributaries or return flows from Clark Canyon Reservoir to Twin Bridges, MT

3) Quantifying the volume of water associated with each identified tributary, diversion, and return flow for the 2004 irrigation season. An additional goal, to aid in identification of major diversions and sources of flow, was to locate by GPS and create a map of significant irrigation diversions, return flows, and tributaries between Barrett’s Diversion Dam and the confluence of the Beaverhead River with the Ruby River at Twin Bridges, MT.

Figure 1. Approximate locations of MSU established flow monitoring stations within the Beaverhead River watershed.

With the exception of the USGS and Bureau of Reclamation stations, an Aquarod or Tru-track was installed in a stilling well at each monitoring site. Aquarods and Tru-tracks log water level and water temperature on a continuous basis and were set to record stream height and water temperature every 30 minutes.

Beginning April 14 th and throughout the summer, gauging sites were visited on an approximate bi-weekly basis. At all gauging sites, flow was measured using a Marsh-McBirney Model 200 Flo-Mate portable flow meter. Flow measurements made with the flow meter were correlated with Aquarod and Tru-track stage height measurements to develop rating curves for the water height measurements collected by the Aquarods and Tru-tracks. Rating curves were developed for each gauging site which provides a means to convert stage height (surface water elevation) to a flow value.

The period of record for most gauging stations ran from May 12 to October 12, therefore, the period of record reported in this study runs from May 12 to October 12, 2004. Clark Canyon Water Supply Company began diverting water on April 26, 2004 and began storing water in Clark Canyon Reservoir (releasing only minimal flows of 27 cfs) from the Clark Canyon Dam on September 10, 2004.

Observations and Measurements

Flow Measurements

Calculations made were total flow in acre feet, average daily flow in cubic feet per second (cfs), and average daily flow in miner’s inches for the period of record (May 12 to October 12) and are illustrated in Table 1. Note that these values are calculated as cumulative seasonal flow values and thus represent only a numeric average. Examination of continuous records of flow at each station illustrates the daily and seasonal fluctuations.

Acre-feet of water diverted indicates water diverted through canals to the Clark Canyon Water Supply Company signors. Acre-feet of water from tributaries and at Beaverhead stations indicates water contributed by natural tributaries or direct return flows (Red Rock River and unknown, Horse Prairie Creek, Grasshopper Creek, Stone Creek, California Slough, Spring Creek, and Ruby River) and water flowing in the Beaverhead River at USGS, BOR, and MSU stations (Clark Canyon inflows and outflows, Beaverhead River at Barrett’s, Beaverhead River at Dillon, Beaverhead River at Point of Rocks, Beaverhead River at Silverbow Bridge, and Beaverhead River at Madison County Fairgrounds). Additionally, Poindexter Slough back into the Beaverhead is categorized in Table 1 as a tributary. Tributary and Beaverhead flows may, in part, consist of return flows but we are not able to distinguish return flows from natural flow with the 2004 data set.

Figure 2 illustrates hydrographs of average daily flow (cfs) for all Beaverhead River stations over the period of record. The Beaverhead River near Twin Bridges (Point of Rocks) and at Silverbow Bridge behaves relatively similarly, suggesting few major diversions or tributaries exist between these locations.

Table 2 indicates that all sections of the river, besides the Beaverhead at Barrett’s to Beaverhead at Dillon section, are gaining more water than is lost through diversions. The greatest unknown sources of water occur between Silverbow Bridge and Madison County Fairgrounds, indicating that the lower Beaverhead River receives return flows and/or is a gaining reach. From Barrett’s to Dillon, the Beaverhead loses 19,568 acre-feet of water through gauged diversion, and an additional 18,009 acre-feet through un-gauged diversions or through deep percolation. Note that flow estimates, reported in Table 1 and used to conduct calculations in Table 2, are not without error. Possible sources of error or disparity include:

• Equipment calibration
• Rating curves
• Unknown sources or diversions of water
• Inaccurate flow readings caused by moss

Mapping

In August of 2004, the river was floated from Barrett’s diversion dam to Twin Bridges. All observed tributaries, return flows, irrigation diversions, and domestic or agricultural pumps were located with a Trimble GPS. All locations were downloaded onto topography maps and are available in Arc View format. Additionally, photos were taken of each location and are linked to the Arc View map, along with approximate dimensions of each tributary or diversion.

Through floating the river, significant tributaries/return flows to the Beaverhead were observed between Silverbow Bridge and Twin Bridges. These flows are, in part, water coming into the Beaverhead from the Big Hole River.

Monitoring flow at 22 locations within the Beaverhead River watershed during the 2004 irrigation season indicated that the largest portion of water diverted from the Beaverhead River occurs between Barrett’s diversion dam and Dillon, and that the greatest return of water to the river occurs below Silverbow Bridge (and below the Clark Canyon Irrigation project). The difference in flow between Barrett’s USGS station and the Dillon USGS station will most likely be greater in years when water is diverted to the East Bench Canal.

Table 2 indicates that all sections of the river downstream from Dillon are diverting water, but gaining enough water (either through return flow, natural tributaries, or groundwater recharge) to maintain relatively similar flows through these sections. However, the average seasonal flow (Table 1) does decline as one progresses downstream from Barrett’s diversion dam to Silverbow Bridge, indicating that, as a whole, the river is continually losing water as it flows through the Clark Canyon Irrigation project.

A significant volume of water is picked up in the Beaverhead between Silverbow Bridge and Twin Bridges. This water is composed of water from the Ruby River, Spring Creek, California Slough, the Big Hole River, and possibly from return flows from the Clark Canyon Irrigation Project.

Based on Beaverhead float trip observations, water budget calculations, and on-site experience from 2004, it is recommended that additional steps be taken in 2005 including:

Additional monitoring stations should be installed to obtain a more complete water budget.

• To improve rating curves and flow estimates for the Beaverhead into Poindexter Slough station and the Poindexter Slough back into the Beaverhead station, efforts should be made to obtain multiple flow measurements prior to significant moss growth in the spring and additional flow measurements following significant moss growth in the summer. This may allow for the development of separate rating curves for pre and post moss growth, thereby facilitating more accurate flow estimates.
• Rating curves should be developed for staff gages at all locations
• All equipment should be installed and operational by April of 2005 and should continue operating until November 2005. Irrigation typically begins in the third week of April and ends in the last week of September. Maximizing the monitoring period will allow for full accounting of irrigation diversions, and may provide some insight into return flow and base flow patterns.

This is an ongoing project through the 2005 irrigation season. Additional observations made in 2005 will be available at this website in winter of 2005. Comments, questions, concerns may be addressed to Holly Sessoms (hsessoms@montana.edu) or Kim Hershberger (kimr00@hotmail.com).