ScholarWorks at University of Montana

The removal of Milltown Dam commenced in 2006, after a century of impacts to the watershed. The last vestiges of the dam were extracted from the confluence of the Blackfoot and Clark Fork rivers in 2009. The dam that once annually blocked the migrations of 10’s of thousands of fish, limited downstream fish movements, created a reservoir that fostered illegally introduced northern pike and was a source of heavy metal inputs to the river below is gone and over 2 million cubic yards of sediments removed. The removal of the dam reversed many of its effects immediately. Connectivity was restored for all fish species and the benefits to other wildlife occurred in months. Many of the changes in the coming years will be subtle, like offering populations more resilience, and promoting the expression of life history tactics that were formerly selected against. However, we have documented drastic local changes in species composition, fish densities, and unimpeded fish passage. Monitoring these changes has occurred on a watershed scale and the activities range form in situ bioassays to population and community monitoring. Dam removal may not ameliorate all the challenges to native fish conservation; however, it may ultimately be the only way to guarantee success. The effects of dam removal are permanent and at a scale which could not be afforded by engineering solutions.

The upper Clark Fork River is the focus of a multi-decade Superfund effort to remediate and restore the watershed from the impacts of historic mining activities near its headwaters. One of the primary goals of the restoration effort is to improve fish populations in the mainstem Clark Fork. Current salmonid population estimates in the Upper Clark Fork average only about 20 percent of the populations in reference rivers in western Montana, and are dominated by non-native trout species. While remediation and restoration of the mainstem river will improve the water quality and habitat of the Clark Fork, additional work to reconnect and restore tributaries is key component of the overall plan to restore fish populations and in particular, migratory native trout species. Trout Unlimited and the Clark Fork Coalition are working with a variety of partners including private landowners; local, state and federal agencies; and other NGOs to reconnect and restore tributary habitats for migratory westslope cutthroat and bull trout populations in the upper Clark Fork watershed. Our assessment work has identified numerous barriers to upstream fish passage including irrigation diversion dams, road crossings, and dewatering. In addition, we have identified the entrainment risk to out-migrating adults and juveniles that may be trapped in irrigation ditches while moving downstream. Work is now underway to prioritize, develop, and implement projects on key tributaries intended to mitigate these challenges and improve native fish populations in both the mainstem Clark Fork River and tributary streams. This presentation highlights the last five years of those efforts.

The Lower Clark Fork River in northwest Montana is an area characterized by lower elevation tributaries, and has been impacted by both natural and anthropogenic effects. In 1999, as Avista Corporation renewed their license for operation of two major hydroelectric facilities, stream restoration was identified as a major mitigation activity. Since then, more than 50 stream rehabilitation efforts have been attempted in approximately 15 drainages with varying degrees of success. Goals of these projects have ranged from bank stabilization and fish habitat enhancement to sediment and temperature reduction. Future efforts will focus on areas with a high probability of success for native species.

During relicensing of its Clark Fork River dams, Avista Corporation cooperatively developed the Native Salmonid Restoration Plan (NSRP) to address mitigation actions for the ESA listed bull trout (Salvelinus confluentus). The NSRP identified aspects including: fish passage, genetics, non-native species, and habitat, among others, to be addressed in an adaptive management progression. Upstream passage began in 2001 with bull trout captured below Cabinet Gorge Dam being radio tagged and released upstream. Subsequent development of a genetic baseline facilitated transport to the region of origin, and parentage analysis confirmed contribution of upstream transported bull trout. Downstream juvenile bull trout passage by tributary trapping and transport was assessed using PIT tags and tributary antenna arrays. Mitigation programs also addressed habitat and non-native species concerns from rearing habitats to Lake Pend Oreille. Population modeling included a risk assessment of trapping and transporting resident bull trout and incorporation of over 10 years of data into a strategic modeling exercise to assess what mitigation actions have worked and where to focus future efforts. Such direction will become increasingly important in the face of dynamic anthropogenic impacts, climate change, and other challenges.

The Clark Fork and Kootenai River basins lie within the Northern Intermontane Basin physiographic province. The intermontane basins are structurally downdropped relative to the surrounding mountains and filled with basin-fill deposits. Groundwater occurs within the unconsolidated to poorly consolidated Tertiary and Quaternary basin-fill deposits, and to a lesser extent in the fractured bedrock in the surrounding mountains. More than 70,000 wells within the Clark Fork/Kootenai River basins withdraw water from aquifers found in unconsolidated to poorly consolidated deposits and surrounding fractured bedrock. Most wells (92 percent) provide water for domestic purposes, but account for only 9 percent of annual groundwater withdrawals; irrigation, public water supply, and industrial wells—only about 8 percent of wells—account for 90 percent of withdrawals.

There are about 194 permitted dams within the Clark Fork Watershed in Montana alone; 51 of these are classified as high hazard. Congruent with state and federal guidelines, all of these high hazard dams are required to have an Emergency Action Plan (EAP) that includes dam breach inundation mapping. The purpose of having an EAP is to protect public safety. Unfortunately, many of these EAP’s are outdated and contain inaccurate data.

Historically, one-dimensional hydraulic modeling software, such as HEC-RAS, has been the primary method to model dam breach inundation. One-dimensional models have yielded good results, but can be challenging to construct and have limitations for modeling flow. Two-dimensional modeling software has attracted significant attention because it permits a more detailed and accurate representation of flow. 2-D modeling has been accepted as an accurate and efficient means to calculate and predict conditions under various flow events. In many cases this is the only approach that accurately calculates resulting flow conditions. In addition, this approach has allowed modelers to provide a clearer picture of the potentially devastating effects of a dam breach through Google Earth^TM animations, and spatially variable depth and velocity distributions. Despite the acceptance of two-dimensional modeling as an accurate and efficient methodology, only two of the 51 high hazard dams within Montana’s Clark Fork Drainage use this technology.

This presentation will give an overview of the dams within the Clark Fork Drainage, explain the differences between two-dimensional and one-dimensional hydraulic models, and then will present examples where 2D modeling was used. These examples include inundation mapping exhibits and Google Earth^TM animations generated for dams within and near the Clark Fork Watershed. This presentation will also briefly touch on the benefits of two-dimensional modeling to other hydraulic structures where detailed velocity distribution exhibits can be used to predict scour, fish passage and habitat conditions.