Trihydro is assisting the Wyoming Water Development Commission (WWDC) in designing and constructing a 9,000 acre-feet reservoir in the Nowood River Watershed, providing permitting, design, and project management services for the new Alkali Creek Reservoir. 

The Wyoming Water Development Commission (WWDC) contracted with Trihydro to perform permitting and design work for a new reservoir in the Nowood River Watershed near Hyattville, Wyoming. Once constructed, the Alkali Creek Reservoir will provide irrigation and recreation opportunities for the surrounding community.

The new reservoir, located off-channel, will be filled with water from Paint Rock and Medicine Lodge Creeks by expanding the existing Anita and Anita Supplemental Ditches. The reservoir will have a total capacity of nearly 9,000 acre-feet, with 6,070 acre-feet serving as irrigation supply and 2,900 acre-feet providing habitat, conservation, and recreation opportunities.


Landowners in the Nowood River Watershed first approached the Wyoming Water Development Office with concerns regarding late-season water shortages and associated watershed impacts. The landowners, then referred to as the Proponents of Nowood Drainage Storage, requested funding from the Wyoming Water Development Commission (WWDC) for a Level I Watershed Study. A Level I Storage and Watershed Study was completed, and results supported assertions by landowners and producers that hydrologic timing caused late-season water shortages throughout the Nowood River Watershed, while also making water available for storage during spring runoff. Based on the Level I Study conclusions and recommendations, the Proponents of Nowood Drainage Storage formed the Nowood Watershed Improvement District (NWID) and requested a Level II, Phase I Study to evaluate Nowood River Watershed water demands; the location, magnitude, and timing of water availability and shortages; and potential storage sites. 

Feasibility Study

Trihydro performed the Level II, Phase I feasibility study and conceptual engineering analysis of potential reservoir storage opportunities within the Nowood River Watershed. The Level I and II studies provided the technical basis for establishing the purpose and need for developing reservoir storage.

Trihydro, in association with its team partners, modeled surface water shortage and availability for storage, identified potential storage sites, and evaluated potential storage site characteristics to determine the most viable storage sites in the Nowood River Watershed.  

Work included developing a StateMOD watershed hydrologic model to simulate physical and available flows as well as shortages in the Nowood River Watershed. The model identified available flows (i.e., flows not currently put to beneficial use under existing water rights) for development or storage throughout the Nowood River Watershed. Work also expanded the evaluation of potential reservoir sites suited for storing available flows so the flows could be released to meet late-season shortages.

The Level I Study identified 35 potential storage sites. During Level II, Phase I work, Trihydro identified five additional sites and evaluated each to determine which might provide the most benefit with the least impact (economic, environmental, or otherwise). To create a short list of sites that appeared to provide the most overall value to the Nowood River Watershed, Trihydro conducted and oversaw more detailed investigations, which involved:

  • Site visits to evaluate geotechnical and geological conditions and assess environmental considerations
  • Archeological investigations
  • Discussions with landowners and agencies
  • Developing conceptual level designs and cost estimates

After evaluating the characteristics, benefits, and impacts associated with each of the 40 identified potential storage sites, two reservoir sites moved forward for more in-depth evaluation, which included an enlargement to an existing dam and a new reservoir on Alkali Creek, a small tributary to Paint Rock Creek (a primary tributary to the lower Nowood River).  

Environmental Permitting

Situated partially on lands managed by the Bureau of Land Management (BLM), the Alkali Creek reservoir site involves Waters of the United States and, therefore, requires a BLM-issued Right-of-Way permit and an individual United States Army Corps of Engineers (USACE) Section 404 Clean Water Act (CWA) permit. Trihydro submitted a final Right-of-Way Application to the BLM on behalf of the WWDC, which triggered the National Environmental Policy Act (NEPA) process by the BLM. Trihydro supported the WWDO during the NEPA process by providing professional engineering and technical services, including:

  • Updating the reservoir and ancillary facilities conceptual design
  • Coordinating and completing surveys to address data gaps
  • Supporting public and cooperating agency meetings
  • Responding to technical questions from the BLM and the NEPA team
  • Preparing a Reservoir and a Road Plan of Development (POD)
  • Coordinating cultural and paleontological surveys
  • Performing wetland and aquatic resources, bat, and Ute Ladies’ Tresses surveys
  • Evaluating alternatives to reservoir development, including groundwater resource development and implementing conservation measures, as well as sub-alternatives related to water delivery and reservoir operations

The site received a positive Record of Decision from the BLM. The final Environmental Impact Statement (EIS) required a water quality Adaptive Management Plan (AMP) to comply with CWA Section 401 provisions, applicable state surface water quality standards, and total maximum daily loads. The AMP includes a plan to monitor water quality, evaluate impacts on water quality, determine project-related impacts, and identify potential adaptive measures or actions to mitigate water quality impacts. The first steps of plan implementation have included pre-construction monitoring in coordination with the local conservation district.

Additionally, Trihydro developed stream and wetland compensatory mitigation plans (CMP) to satisfy USACE Section 404 CWA permit requirements. Work included assessing potential stream and wetland impacts, identifying stream and wetland mitigation sites, and developing compensatory mitigation designs. Trihydro used the Wyoming Stream Quantification Tool (SQT) to quantify stream impacts and assess potential stream restoration designs. Trihydro coordinated and communicated with permitting agencies, project sponsors, and landowners to develop the best mitigation approach to satisfy permitting requirements. The site received a positive Record of Decision from the USACE.

As required by the issued USACE 404 CWA permit, Trihydro is supporting coordination between WGFD and NWID in developing a Memorandum of Understanding (MOU) for future reservoir flow operations. A stream flow AMP is being prepared to support the MOU and describes monitoring and an adaptive management approach to reservoir flow operations.   

Ecological Surveys

A key component of navigating the NEPA process involved completing ecological surveys. To develop the EIS, Trihydro performed an aquatic resources inventory to delineate aquatic resources including wetlands, streams, and other non-wetland surface waters such as ponds and irrigation ditches that may be affected by the project. Trihydro also completed ecological surveys for sensitive flora and fauna species, including the Ute ladies-tresses orchid (ULT) and bats. 

Aquatic Resources Inventory

Trihydro delineated wetland boundaries along Alkali Creek, within the 295-acre proposed reservoir boundary, using data collected from field sample locations. Trihydro also conducted wetland delineations and aquatic assessments along Alkali Creek downstream of the proposed reservoir embankment to its confluence with Paint Rock Creek, and along the Anita and Anita Supplemental ditches. Trihydro biologists used the following methods in delineating aquatic resources:

  • Desktop assessment of potential wetlands using a combination of National Wetland Inventory (NWI) data, National Resources Conservation Service (NRCS) soils data, topographic maps, and aerial imagery
  • Assessment of 41 paired “determination points” in wetland and adjacent upland areas to characterize wetlands based on methods described in the USACE 1987 Wetlands Delineation Manual and the 2008 Arid West Region Supplemental Manual
  • Characterization of each “determination point” based on the presence of wetland indicators including (1) hydrophytic vegetation, (2) hydric soils, and (3) hydrology
  • Mapped boundaries of each unique aquatic resource using a sub-meter GPS unit

Streams and ditches were delineated based on the average measured width between ordinary high water lines on each bank

Altogether, Trihydro delineated approximately 25 acres of aquatic resources (12 acres of wetlands and 13 acres of open water stream or ditch). Trihydro incorporated aquatic resources inventory results into a comprehensive report and submitted the report to USACE to provide an accurate assessment of resource boundaries, acreages, and potential impacts to be assessed during the NEPA assessment of the project.

Bat Acoustic Survey

Trihydro used the following methods to perform bat surveys to identify the presence of BLM-sensitive bat species including the spotted bat and western long-eared bat:

  • Delineating potential bat roosting habitat including rock outcrops, rock crevices, caves, structures, and large trees with loose bark and/or cavities.
  • Establishing seven acoustic monitoring stations near potential roosting habitat.
  • Active acoustic monitoring using a Pettersson MD-500 microphone combined with Sonobat Live and Bat Recorder active recording software.

Trihydro analyzed over 1,000 potential bat vocalizations recorded onsite using Sonobat 4.2.1 (bat call analysis software) with the Western Wyoming Classifier. Trihydro then manually vetted the bat acoustic data to verify or confirm species presence based on diagnostic call characteristics.

Through acoustic survey data, Trihydro biologists verified the presence of eight bat species including the pallid bat, big brown bat, silver-haired bat, western small-footed myotis, little brown bat, long-legged myotis, spotted bat, and western long-eared bat.

Based on habitat assessment and acoustic survey results, Trihydro biologists concluded that:

  • Spotted bat occurrence in the project area is likely due to foraging opportunities rather than roosting
  • Potential roosting habitat for the western long-eared bat is present but limited to a small number of large cottonwood trees along Alkali Creek at the southern extent of the project area.

The results from these surveys were incorporated into a comprehensive report and accepted by the BLM.

Ute Ladies-Tresses Orchid Survey

Trihydro performed ecological surveys for ULT, a federally listed endangered species, to identify potential habitat and the presence of any individuals or populations.

During the ULT flowering period, Trihydro completed pedestrian surveys within the 295-acre proposed reservoir boundary, along Alkali Creek, downstream of the proposed reservoir to its confluence with Paint Rock Creek, and along the Anita and Anita Supplemental ditches. Trihydro used the following methods in performing ULT surveys:

  • Desktop assessment of mesic habitats using a combination of NWI data, NRCS soils data, topographic maps, and aerial imagery to guide field surveys
  • Habitat suitability assessment based on methods described in USFWS 1992 Interim Survey Requirements for Ute Ladies’-tresses Orchid and USFWS 1995 Recommendations and Guidelines for Ute-Ladies’-tresses Orchid
  • Walking 9 miles of streamside wetland habitat in search of ULT stalks

Trihydro did not identify ULT individuals or populations during the survey and the negative data was submitted to BLM to provide an accurate assessment of impacts, or lack thereof, on ULT during the NEPA assessment. For rare plant species surveys, negative data is often considered as valuable as data indicating species presence to build a better understanding of species distribution and habitat associations.

Design Work

Trihydro is currently developing final designs for the new reservoir, including supply canals, diversions, intake structures, spillways, outlet works, control buildings, public facilities, downstream enhancements, and delivery systems. To further support reservoir development, Trihydro conducted and coordinated additional investigations and designs, including wetland design, stream mitigation using the SQT, and geotechnical investigations (performed by a specialty subconsultant) to assess site foundation conditions and suitable embankment construction borrow materials and associated hazards.

During the design process, Trihydro conducted topographic surveys using aerial LiDAR and unmanned aerial systems (UAS) photogrammetry along the existing and proposed dam site, spillways, and irrigation ditches. Trihydro also surveyed structures such as bridges, culverts, irrigation infrastructure, and diversions to assist in the final design. Trihydro also completed legal boundary surveys to support the Record of Survey, the establishment of rights-of-way, easements, and land purchase.  

Final dam designs, including a new 100-feet tall by 2,500-feet long earthen embankment, outlet works capable of delivering 200-225 cubic feet per second (cfs), service spillways, diversions, enlargements to 4.3 miles of existing irrigation ditches, and channel stabilization structures along Alkali Creek are being completed. The final designs will include provisions for public access, including a 1.4-mile public access road, a parking area, and a boat ramp. Construction is scheduled to begin in the next few years. Following construction, NWID will operate and maintain the reservoir. 

Legal and Construction Permitting

Concurrent with design work, Trihydro and teaming partners assisted NWID in land negotiations to enlarge irrigation ditches and property that would be inundated by the reservoir. Work included appraising the properties for current market value, negotiating compensation, developing easement and purchase area exhibits and easements, and recording with the local County Assessor. Trihydro has also been completing and submitting permits required by Big Horn County, the Wyoming Department of Transportation (WYDOT), and the Wyoming State Engineers Office (SEO) to prepare for construction.  Permits include water rights; multiple floodplain, right-of-way, and access permits; and a landscape permit. 

From environmental permitting and ecological surveys, to design and construction preparation, Trihydro has supported the multi-year process involved in creating a new reservoir. 

Want to keep reading?