A key feature in our balloon aerial photography is the distribution of macrophytes (rooted aquatic vegetation) within the Ranch section of the Henry's Fork. Macrophytes are a vital component of a healthy river ecosystem and provide essential trout habitat. Jim Gregory summarizes and evaluates all the available research on this subject so comprehensively in his 2008 Caldera Report that my regurgitation of the information would be a waste of time. Instead, key excerpts from the report’s section on macrophytes are pasted below. Anyone interested in the Henry’s Fork should definitely check out the full 80 page document. Visit http://www.henrysfork.com/Caldera%20Project/caldera.php to download the complete pdf.

Change over time
Macrophyte (rooted aquatic plants) abundance and species composition in the Henrys Fork have changed substantially over the 1958 – 1996 period of recorded studies (Shea et al. 1996).Macrophytes in the Henrys Fork were divided by Shea et al. (1996) into groups based on morphology. Group 1 species were tall, robust, and erect species that thrive in low water velocity, silt – rich, high nutrient environments (Potamogeton. pectinatus, P. richardsonii, Elodea canadensis, and Myriophyllum exalbescens). Group 2 species were shorter, bottom dwelling species that are more tolerant of higher water velocities, prefer higher light intensity, and are capable of rapidly colonizing disturbed sites (Callitriche hermaphroditica, Ranunculus aquatilis, and Zannichellia palustris).

By 1977-1980, after winter flow management had changed (See section IX.a) and was more conducive to macrophyte growth than in previous periods, Group 1 species comprised > 95% of the biomass, while Group 2 had declined to trace quantities. Prolific beds of lush vegetation reached the water surface throughout much of the river. Biomass measurements and descriptions indicated that during the late 1970s, the plant community likely attained its peak biomass for the 1958 -1995 period.

The Decline
While all species present in the 1970s and 1980s were still present in the Henrys Fork in 1996, they were greatly reduced in biomass, and individual plants were often unusually small (Shea et al. 1996).

The declines in macrophyte abundance occurred over a relatively long period of time, possibly beginning as early as 1979 and with abrupt declines in the late 1980s (from 1989 – 1990 ) and again after 1992. The cause of these declines is not clear, but Shea et al. (1996) indicated that several factors could contribute, including silt deposition from releases at Island Park Dam in 1979 and again in 1992, low winter flows and associated ice formation and breakup, high spring flows and associated scouring, and grazing by waterfowl. Additionally, reductions in macrophytes, particularly Group 1 macrophytes, cause what Shea et al. (1996) referred to as a “negative feedback loop” where macrophyte density is reduced, which causes increased water velocity, causing silt substrate in which other macrophytes are rooted to be scoured, which further reduces macrophyte density. Shea et al. (1996) also identified conditions which perpetuate the conditions that reduce Group 1 species and favor Group 2 species. These conditions include reduced macrophyte biomass, unstable fine sediments, and waterfowl grazing.

Ecological Importance
This reduction in macrophyte biomass/density altered many facets of the river’s ecology, including channel roughness, flow velocity, water depth, patterns of sediment erosion and deposition, nutrient cycling, habitat quality and availability for fish and invertebrates (see section X.k), and food supplies for wintering waterfowl (Shea et al. 1996). Most notably for fish, changes in macrophyte density directly affect water depths (Vinson et al. 1992 ) and winter habitat for juvenile trout (Griffith and Smith 1995). Macrophytes roughen the channel, causing resistance to flow and therefore increasing water depths. Vinson et al. (1992) showed that in the Henrys Fork water surface elevation dropped 1 – 1.6 ft ( 0.3 – 0.5 m) at a constant discharge as macrophyte coverage of the channel decreased from 60 – 90% of stream-bottom coverage to near 0%. This caused an associated 40 – 78% increase in water velocities and a 40 – 79% decrease in channel cross-sectional area. Since water depth is a habitat component for trout, both during summer and winter, these reductions in water depth and cross-sectional area reduce fish habitat availability and ultimately, at least locally, fish populations.

Reductions in macrophyte abundance have also been seen to cause juvenile trout to emigrate from wintering areas as these reductions in abundance occur (Griffith and Smith 1995). If macrophytes can provide cover for juvenile trout through the winter, as they have been seen to do in the Last Chance Canal (see Section X.c) and Chick Creek (Griffith et al. 1996, re-establishment of macrophytes could cause a huge increase in survival of juvenile trout and ultimately increase the trout population throughout the Caldera Section.

Restoration Efforts
Efforts to reduce impacts to macrophytes have included waterfowl hazing, which occurred throughout the 1990s, and transplanting of swans, which was last done during the winter of 2004 -2005 (L. Hanauska -Brown, Pers. Comm. Idaho Department of Fish and Game). Additionally, the fishing season in the Railroad Ranch, which closed on 30 September as early as 1954, was extended to close with the general season on 30 November beginning in 2004. This change in season length was intended to encourage human disturbance of the Railroad Ranch area which, it was hoped, would result in passive hazing of waterfowl and cause them to continue fall migrations to other wintering areas.
Returning macrophyte populations to the density and species composition of the 1970s will be difficult. However, Shea et al. (1996) suggested management actions that should aid in the recovery of Group 1 species. These included higher, stable winter flows, reduced variation between winter and early peak flows, delaying peak flows until June or later (which would shift peak flows even further from the natural flow regime –see Figure 2), and reducing waterfowl herbivory. The most important of these may be reducing waterfowl herbivory. The number of swans counted in mid-winter surveys has increased substantially since the first surveys were conducted in 1972. This increasing swan trend has also been observed in the Henrys Fork (Van Kirk and Martin 2000), which is expected, as it is not unusual for 40% of the tri-state Rocky Mountain
Population of Trumpeter Swans (Idaho, Wyoming, and Montana) to be on the Henrys Fork during the winter (Vinson 1990).

Source: Caldera Report, Gregory Aquatics 2008