Case Study Site

Natural Resources Conservation Service (NRCS)

3

The Henry’s Fork watershed is large, complex,and picturesque. Over 800,000 acres of theupper watershed are in public ownership.Elevations range from 10,240 feet at TargheePeak to near 4,500 feet at the confluence of theTeton and Henry’s Fork Rivers (Figure 1).Precipitation varies from 40 inches in the upperwatershed to 10 inches in the case study site. Inthe lower watershed, most of the precipitationfalls in May and June.

Forest and range land predominate in the upperwatershed. Rolling loess hills and basaltic plainsdefine the lower reaches of the watershedsupporting row crop agriculture and ranching.This working landscape, the site for the casestudy, is populated by scattered farms andranches supported by small, rural communities.Agriculture, ranching, and tourism are mainstaysof the regional economy (Figure 2). The casestudy site corresponds with the area beingresearched by TRLT and its conservationpartners.

The case study will focus on the Henry’s Forkriparian corridor, located within the landscapecontext of the lower reaches of the river. Thisarea was selected for more detailed analysis becauseit is experiencing some of the most intensedevelopment pressure in the watershed. Thedetailed site is two miles wide (approximately onemile from the top of the bank on either side ofthe Henry’s Fork River) and approximately 40miles in length (Figure 3). In some locations, thestudy corridor was widened to include habitatsor other resources of importance as determined

Figure 1 - Location map.

Figure 2 - The lower Henry’s Fork watershed is a workinglandscape producing cattle, wheat, potatoes, and largerainbow trout.

Figure 3 - The lower Henry’s Fork and its main tributaries.

Cra

ig J

ohns

on

4

by the research team. The study corridor beginsat the confluence of the Henry’s Fork and WarmRiver downstream to the confluence with theTeton River. The study will also include the FallRiver and Conant Creek--major tributaries of theHenry’s Fork.

There are three distinct segments to the Henry’sFork within the case study corridor. The uppersegment extends from the Warm Riverconfluence downstream to Ashton Dam. Anincised river basin characterizes this segment--steep side slopes bounded by gently slopinguplands (Figure 4). Soils in this segment(Marytown-Robinlee and Greentimber) are deep,well-drained of loess origin underlaid by glacialdeposits. In some locations underlying basalt isexposed. Pre-Anglo settlement upland plantcommunities were dominated by grassesincluding wheatgrass (Agropyron sp.), needlegrasses (Stipa sp.); and Idaho fescue (Festucaidahoensis); shrubs including sage (Artemisia sp.),service berry (Amelanchier alnifolia), bitterbush(Purshia tridentata), and rabbitbrush,(Chrysothamnus sp.) Along this segment ofthe Henry’s Fork, the riparian zone is narrow withoccasional stands of river birch (Betulaoccidentalis), alder (Alnus incana), willow (Salixsp.), and chokecherry (Prunus virginiana). Sedges(Carex sp), rushes (Juncus sp.), and grassesdominate the banks. Old inactive point bars anddeltas are vegetated by xeric shrub-steppe plantcommunities.

The middle segment runs downstream fromAshton Dam to the water diversion below St.Anthony. In this segment, the river channel istypically less than 10 feet below the adjacentuplands. Several large side drainages intersectthe Henry’s Fork in this segment (Figure 5). Soilsinclude a segment of those described above andRexburg-Ririe and Kucera soils, which are deep,well-drained, and formed in loess. Below the FallRiver confluence, the soils including St.Anthony- Allewitt and Eginbench, which aredeep with drainage varying from well-drained to

poorly drained. Basalt formations are evident insome locations. Pre-settlement upland vegetationwas similar to that described above. The riparianzone remains narrow until just above St. Anthony.Rushes, sedges, and grasses are dominant groundcovers with occasional stands of Cottonwood(Populus trichocarpa), box elder (Acer negundo),willow, river hawthorn (Crataegus douglasii), andchokecherry.

The lower segment is characterized by a broadfloodplain and sinuous channel alignment. Theriver channel is braided in many locations withactive point bars, bank erosion on the outsidebends, and numerous oxbows (Figure 6).Cottonwoods dominate the plant communitywith occasional large stands of river hawthorn,willow, dogwood (Cornus sericea), andchokecherry. Plant species diversity andcommunity structural diversity is highest in the

Figure 4 - The Henry’s Fork flows through an incised canyonat the northern end of the study site. The inactive point barhas reverted back to shrub-steppe habitat.

Figure 5 - The middle segment of the Henry’s Fork flowsthrough range land and irrigated pasture.

Cra

ig J

ohns

on

Dav

e Be

ll

5

The Henry’s Fork, itstributaries, and relatedcanal system function ashabitats and corridorsfor migration and

dispersal for a variety of wildlife species. In coldshrub-steppe landscapes like the case study site,riparian corridors and wetlands are among themost important habitat types. Over 80% of allvertebrate species use these habitat types at leastonce during their life cycle. The Henry’s Forkcorridor may be the key to biodiversityconservation in the lower segment of thewatershed.

lower segment. In the first two miles below theSt. Anthony diversion, cottonwood stands are insenesance with little recruitment of youngcottonwoods. Numerous irrigation canals extendinto the agricultural matrix and function asephemeral riparian corridors (Figure 7). Soils inthe lower segment are deep with variabledrainage, the same general soils groups that arefound downstream of the Fall River confluence.

Henry’s Fork flow rates through the 40-mile studycorridor are regulated by management of twoupstream dams and Ashton Dam (see Figure 1).In addition, several water diversions affect riverflow during the growing season. Upland irrigationaffects groundwater levels, and return subsurfaceflows to the river result in occasional seeps andsaturated bank soils.

Figure 6 - The Henry’s Fork is broad, frequently braided, andhas an extensive flood plain as it exits the southern end ofthe study site.

Figure 7 - Canals, roadsides, and other introduced corridorsprovide wildlife habitat and connectivity in the lower Henry’sFork Watershed.

Han

k H

enry

Jare

d B

arne

s

Cra

ig J

ohns

on

Cra

ig J

ohns

on