Silver Creek Enhancement Plan Complete

The Silver Creek Enhancement plan developed by Ecosytem Sciences is complete! It is available digitally on savesilvercreek.com.

Copies for check our are available at the preserve and in the Hailey office as well (116 1st Ave. N. Hailey, ID 83333). Please email or call Dayna Gross with questions or concerns, dayna_gross@tnc.org (208-788-7910). The Nature Conservancy will be developing our implementation strategies and a plan summary in the next few weeks and those will be available on line and at the offices as well.

A History of Fish Abundance & Compostion in Silver Creek, Idaho

Pre-Settlement Period
According to testimonials from travelers passing through the area, Silver Creek was about 25 feet wide, two feet deep, and so full of trout they could hardly swim. At this time, all trout in the Silver Creek watershed were native redband trout, a variety of rainbow. When all the large marshes were functioning and the watershed was covered with native vegetation, its wildlife and fisheries were highly sought after by Native Americans.

1875 to 1947
With the advent of large numbers of livestock moving into the basin and the soon to follow agricultural practices, the impacts to Silver Creek were evident as early as 1903. Hauk (1947) reported a much wider stream than exists today, with heavily silted tributaries, and a dense trout population compared to other trout streams in the country. Nonetheless, Hauk believed the trout fishery to be in decline, and in response, all the Silver Creek tributaries were closed to fishing from 1934 to 1946. As far back as 1917, Silver Creek was considered by sportsmen to be the most highly productive trout fishery in the country. Even in its more degraded state, Silver Creek and its tributaries (as it is today) supports a valuable and productive trout fishery.

By the 1920s, government agencies were stocking brook trout in the Silver Creek watershed, and by 1947 their numbers made up the highest percentage of trout species in the watershed. From the 1920s to 1930s, McCloud River rainbow trout were stocked in Silver Creek. In later years, the Idaho Department of Fish and Game stocked other varieties of rainbow trout. Unfortunately, these stocking practices are the primary reason that native redband trout no longer survive in the watershed in their pure form.

1947 to 1980
During the 1950s, Silver Creek was intermittently closed to fishing because it was believed that over-fishing was causing the declining trout population. Agricultural reclamation, meanwhile, was eliminating the huge marshes in the tributaries of Silver Creek; however, no data are available to determine the impacts to the trout population from these land conversions. Over the next several decades, various sources reported declines in the fishery. More recently, decreased catch rates and sizes have been documented.

1980 to Date
In the late 1970s, brown trout was stocked in waters with direct access to the Silver Creek watershed. By 1986, brown trout made up 19 percent of the trout population in Silver Creek; by 2004, this figure increased to 60 percent and then leveled off at 55 percent in 2007. As the proportion of brown trout has increased, there has been a concurrent decrease in the proportion of rainbow trout. Due to this indiscriminate stocking, brown trout are here to stay in Silver Creek and are now one of the important trout species in sport fishery.

A 2001 fish population analysis found 2,800 trout per mile in Silver Creek, which is much higher than the numbers found in other trout streams in the country. In fact, trout density (1,573 rainbow/hectare) in Silver Creek was the highest measured for a mixed species salmonid fishery in the United States. Wiley (1977) reported 3 to 6 pound trout were regularly taken by fishermen.

In 2007, IDFG sampled Silver Creek at three locations (Stalker Creek, Cabin, Martin) to evaluate trends in population abundance and structure and estimate rainbow trout and brown trout abundance. Brown trout densities ranged from 308 to 640 fish (>100mm/km) at the Cabin and Martin sites, respectively, while rainbow trout densities ranged from 95 to 1,726 fish (>100mm/km) at the Martin and Cabin sites, respectively. IDFG sampled again in 2010; however, the results will not be available until spring of 2011. Nevertheless, some preliminary observations are:

• The fishery appears to be moving toward a brown trout- dominated fishery ; the upper sections of Silver and Stalker creeks are about 60% brown trout and 40% rainbow, while the lower reaches are about 80% brown and 20% rainbow.
• The shift from rainbow to brown trout dominance is a function of habitat degradation (primarily temperature because browns have a higher tolerance) as well as piscivory.
• Total abundance of trout is not much different from 2007
• Age analysis indicates no year classes have been lost; and, remarkably, some brown trout are 12 to 14 years old.
• Growth rates remain strong, about the same as in previous sampling years
• Species composition has changed with the likely extirpation of mountain whitefish

In 2000, Jack Hemmingway stated that Silver Creek was now better fishing than it was in the 1930s. Brook trout, however, have fewer numbers now than when they were originally stocked over a half century ago. Reports from 1952 to 1997 indicate that fishermen were averaging a catch rate of about one trout per hour. From 2001 to 2007, Gillian (2007) reported a decline in the trout population. It is doubtful that fishing success, per unit of time, in Silver Creek has decreased much over the past 75 years. Trout populations naturally have wide variations in year to year population size and could be the cause of the consistent reports of fish population declines.

In June 1992, the first recorded trout kill occurred at “Point of Rocks” on Silver Creek. This could have been caused by low dissolved oxygen (2.5ppm), high stream temperatures, toxic inputs, a combination of these factors or unknown factors. In June 1994 a second trout kill was reported when dissolved oxygen was 3.2 ppm; however none of these isolated kills had a significant effect on the trout population. Reported fish kills in the Silver Creek watershed are quite rare to date.

Silver Creek- Winter Habitat Conditions

In summer months, biological conditions and forces set the predominate ecological processes in Silver Creek and its tributaries—in winter, however, physical process dominate over biological. Winter is a stressful period for stream-dwelling organisms, especially fish. Low water temperatures slow the rate of digestion and may limit the amount of energy available for metabolism and growth, even if food is available and feeding occurs. Decreased water temperatures reduce the swimming performance of trout, which impairs their ability to escape predators. Angling is closed in Silver Creek during the winter months, but catch-and-release regulations apply in other streams in the basin.

At the onset of winter, juvenile salmonids may form aggregations in open water, especially in thermal refuges like spring inflows, or may conceal themselves in woody debris, in interstices of the substrate, or under undercut banks. Silver Creek and its tributaries offer all these conditions during winter, providing trout with ample winter cover and escapement from predators.

Winter water temperatures in Silver Creek moderate a few degrees in comparison with most trout streams in the region, but are cold enough to induce behavioral changes in trout. Water temperatures are warmer in the upper reaches near spring inflows, and are considerably cooler downstream. Of particular importance in spring-driven systems like Silver Creek is the effect and influences of ice. A floating ice cover can dramatically increase turbulent shear stress on the streambed, thereby causing peak annual sediment-transport events to occur during the breakup of an ice cover or the release of a breakup ice jam. These events often have high discharges, with gouging and abrasion of the bed and banks by moving ice. Ice in a stream channel can reduce the flow areas, increasing under-ice water velocity, scouring bed sediments, and possibly shifting the path of the deepest flow (thalweg). Solid ice is not the only condition that can alter a stream’s structure—frazzle ice can impinge flow against the channel sides, thus contributing to bank erosion.

Ice effects can occur over varying scales or time and channel length. In Silver Creek, icing is more common in the lower reaches below Kilpatrick Pond than in upper reaches, because water temperatures are warmer upstream near the spring sources. At the local scale, an ice cover over a short reach may redistribute flow laterally across the reach, accentuating erosion in one place and deposition in another. Ice may dampen or amplify erosion processes locally. Dampening effects of ice include reduced water runoff from the watershed, cementing of bank materials by frozen water, and ice armoring of bars and shoreline. Amplifying effects include accelerated erosion and sediment transport, notably during the surge of water and ice consequent to the collapse of ice jams.

Surface ice occurrences on Silver Creek are seldom and occur in localized reaches. When Silver Creek experiences ice cover, the stream tends to ice from the bottom-up, by first forming anchor ice (ice sheets attached to the substrate) and then developing frazzle ice in the water column. If low temperatures persist, thin ice sheets eventually form across the channel. While surface ice generally forms for short periods, frazzle ice and anchor ice will persist much longer. Frazzle ice is like pebbles suspended in the water column, and when working against the stream bank, it acts like sandpaper and can cause significant erosion. Stream bank abrasion by icing may explain how undercut banks are formed in Silver Creek since the stream lacks the flow velocities necessary to account for undercutting.

The detrimental effects of ice formation and sediment deposition are evident in some lower reaches of Silver Creek. Sediment deposition reduces channel capacity, and icing increases the stage (height) of the water surface in deposition areas such that stream flow overtops the stream banks in local sites, as displayed in these photos downstream of the Preserve. Sediments in this area cause winter flows to overtop banks, ice then forms on top of the bank undercuts and builds-up until the overhanging bank cannot support the weight of the ice and the stream bank collapses. This results in the loss of the undercut bank and consequently, of valuable fish habitat—it also contributes new sediments to the stream and provides platforms for encroaching reed canary grass, night-shade and other invasive species.

Silver Creek habitat is adversely affected by temperature and sediments in the summer and the winter. Continued sediment inputs and winter icing conditions will have a negative effect on channel morphology, with changes in thalweg depth and location, stream bank erosion and loss of undercut banks. Consequently, although winter is the most favorable period for stream restoration work, sediment disturbance and other potential negative effects must be carefully considered.

Summer Fish Habitat Conditions- Summary of the 2010 Silver Creek Fish Habitat Inventory

A key indicator of a stream’s health is the quality and quantity of habitat. Silver Creek is known for its first class trout fishery. Because a detailed inventory of Silver Creek’s fish habitat has never been performed, one of the tasks Ecosystem Sciences Foundation took on was the identification, qualification and mapping of aquatic habitat in Silver Creek on The Nature Conservancy’s Preserve.

The stream was floated from Stalker Creek to Kilpatrick Pond and dam. Deep runs and pools were surveyed by snorkeling. Habitat inventories focus on the primary types of fish habitat necessary for all trout life stages, including spawning and incubation, early rearing (ER), young-of-the-year (YOY), juvenile and adult habitat. Each life stage has specific habitat requirements that are critical, including gravel size, sediment conditions, instream cover, escapement, pools and run depths.

A habitat inventory is essential in order to define the principal limiting factor(s) on a fishery. For example, while spawning habitat may be extensive in a stream, a lack of early rearing habitat might be the cause of a small adult population because young-of-the-year and juvenile trout are susceptible to predation. In addition to habitat limitations, water quality can also affect a fishery and adversely impact fish production and size.

In the case of Silver Creek, summer temperatures and sediment deposition have been cited as having the potential to impact the trout fishery. What has not been identified, to date, is the extent to which habitat quality and habitat availability factor into the health of the stream’s fishery.

The conclusions that can be drawn from the fish habitat inventory are:

• With the exception of Kilpatrick Pond, all stream reaches surveyed contained some critical trout habitat feature. Spawning habitat with clean gravels is distributed throughout the stream. Early rearing and young-of-the-year habitat is juxtaposed with adult holding/rearing habitat, so that Silver Creek exhibits a mosaic of trout habitat in all reaches.

• Fish production is exceptionally high throughout the surveyed stream reaches. This is because benthic invertebrate (insects that live on the bottom of the stream) production occurs in all reaches in substrate and aquatic vegetation, providing an unlimited food base. Incubation and egg-hatch appears to be very high and redds (depressions on the stream bottom in beds of gravel where fish eggs are deposited) are little affected by sediments. In fact, successful incubation requires a certain amount of sediments to ensure an adequate protective cap develops over the redd.

• Sediment deposition is greatest at the confluence of tributaries and agriculture drains into Silver Creek. These are isolated sites of extreme sediment depth and probably contain sediments deposited years ago as a legacy of livestock grazing throughout the watershed.

• Sediment deposits are relatively thin (< 2 or 3 inches) in other stream reaches. The gravel areas covered by thin layers of fine sediments pose no problem to large trout building redds; they can easily swipe away the fines as the egg nest is dug. These sediments are also too thin to limit benthic invertebrate production.

• Pools, especially deep meander pools, are scoured of sediments. As stream flows enter outside bends, the flow velocity increases, which not only forms the pools but prevents sediment accumulation. Without these physical processes, all of the pools in Silver Creek would have vanished long ago under legacy sediments.

• Upper reaches of the stream from Stalker Creek to the Grove Creek confluence are heavily canopied and banks stabilized with riparian vegetation. The middle and lower reaches of Silver Creek within the Preserve, in contrast, lack the riparian habitat of upper reaches and are widened because of past livestock grazing. These conditions, however, do not significantly degrade instream trout habitat.

• Reed canary grass (RCG)(Phalaris arundinacea) is encroaching in many places along the stream. Typically, reed canary grass begins building platforms on what were once undercut banks. In many places, undercut banks have been lost due to winter conditions exacerbated by sediment inputs. Reed canary grass easily becomes established on these disturbed sites. The long term threat from reed canary grass is in those naturally shallow channel reaches where the plant builds platforms and encroaches into the channel year-by-year.

• Fish passage is generally not an issue within the Preserve boundaries, although a beaver dam about one-quarter mile above the Stalker Creek road does inhibit trout movement into the upper watershed streams (Cain, Mud, Stalker, etc.). IDFG and TNC staff have removed some beaver and pulled down some of the dam, but the dam continues to have a backwater effect that accumulates sediments. The depth of fine sediments in this backwater area negates any positive benefits related to pool habitat; deep sediments have buried both spawning and benthic production values. The dam needs to be brought down to the stream water surface elevation to encourage sediment movement and more efficient trout passage. Except for this beaver dam, the only other impairment to trout migration on Silver Creek is the Kilpatrick Dam.

• Sediment movement into Silver Creek is clearly through the agriculture drains and tributaries. Significant deposition areas occur at these confluences and the solution is to attenuate, to the extent possible, sediment inputs from agriculture lands adjacent to the tributaries and from the irrigation ditches. There are also a few minor places within the Preserve that could generate sediment inputs from overland flow in the spring.

• The only place in Silver Creek that does not support high quality trout habitat and benthic invertebrate production is Kilpatrick Pond. Legacy sediments combined with annual inputs of new sediments have rendered this reach of the stream all but unusable (except at night) for all trout life stages except adult holding. Over time sediment accumulation has progressed upstream to the Loving Creek confluence. Thermal loading in the broad, shallow ponds also restricts trout use. Angling and catch effort continues to be high in the pond—mostly below the bridge and the Preserve boundary—because adult brown and rainbow trout move downstream in response to density-dependent competition, i.e., competing for space with one another. Also, trout feed on scuds and aquatic insects like midges, which are adapted to fine sediment environments.


Results from this fish habitat inventory indicate that the Silver Creek fishery not only lacks an identifiable habitat limiting factor, but habitat throughout the stream supports all trout life stages. Silver Creek is a legendary fishery precisely because of the habitat quality found throughout the stream. Although physical habitat and the food base is not limiting, the fishery is adversely affected by elevated summer temperatures and sediment inputs. As temperature and sediment conditions worsen in time, it can be expected that these conditions will impose a limiting factor(s) on the fishery.

The Silver Creek Fish Habitat maps provided in these links detail the results from the habitat inventory from Stalker Creek (just upstream of the confluence with Cain Creek)to Kilpatrick dam. (Some terms used in the maps include: ER- early rearing, YOY-Young-of-year, RCG-Reed canary grass.)

Saving water through irrigation changes

This week we showcased our first irrigation project focused on water conservation! We installed a variable rate irrigation (VRI) system on the Point of Rocks pivot. We are hopeful this will be the first of many of these VRI systems in the valley.

This is the first of its kind in Idaho, even though the technology was developed at the Unviversity of Idaho twenty years ago. One of the 'tipping points' identified by Bill PLatts at the first enhancement plan meeting was the groundwater level. He said he had seen spring creeks dry up in just ten years from too much groundwater pumping. We need to make sure that never happens at Silver Creek and this may be a useful tool in that effort. Every time the pivot makes a circle (every two days, roughly), with this technology we will be saving over 419,000 gallons of water!! That is 419,000 gallons that can stay in the aquifer for Silver Creek.

Field Visits

Ecosystem Sciences Foundation will spend the next two weeks at Silver Creek. Next week, they will be meeting individually with landowners and natural resource professionals. The following week will be spent in the field verifying the mapping while exploring and studying the creek. Several meetings have already been set, but they do have room in their schedules if you would like to arrange a site visit. Please call Dayna at 788-7910 to set something up.

Preliminary Enhancement Plan Presentation

Picture above- Laura Hubbard, The Nature Conservancy's Idaho State Director talking with John and Elizabeth Stevenson about riparian plantings done along Patton creek last fall.

On June 24th, over fifty people gathered to hear Ecosystem Sciences Foundation present the preliminary enhancement plan for Silver Creek. After the meeting I hung around and spoke with a variety of people. Reactions ranged from staunch excitement to extreme disappointment. ESF stayed their course of ‘do no harm’ and advocated taking care of the remaining sediment inputs before doing any more extensive stream work. Because they think sediment inputs are coming largely from overland flow during the snow melt and runoff events, stopping those inputs will mean creating more substantial riparian buffers. Much of this work has already been done, riparian buffers being the focus of stream work from the 1970s to late 1990’s, but there are still some areas that need attention. Beyond the riparian buffers, we talked about a variety of subject ranging from groundwater usage, extending conservation partners, mitigating temperatures with increased riparian plantings, trout populations and existing (and lacking) data, restoration philosophies, and the list goes on.

It has been my goal from the beginning to have the planning process as open and transparent as possible. Nothing is written in stone at this point! This plan is still very much a work in progress—I myself submitted four pages of questions, concerns, ideas, and input to ESF last week and am waiting for their response. Feel free to do the same, here or in an email or phone call—the more they hear from us, the more robust the plan will be.