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Kilpatrick Pond and Dam Restoration Feasibility Study - Final Report

Prepared for: The Nature Conservancy

March 27, 2007

Prepared By:

Gillilan Associates, Inc.

308 N. Bozeman, Bozeman, MT 59715

406.582.0660

In Association with: Applied Geomorphology

Hoitsma Ecological

Oxbow, Inc.

DTM Consulting

1 EXECUTIVE SUMMARY

This report details an investigation into restoration and enhancement alternatives

for Kilpatrick Pond on Silver Creek. A comprehensive literature and data review

was undertaken to assess historic conditions, trends and current conditions of

Silver Creek on the Nature Conservancy Silver Creek Preserve and represents

the bulk of this report. Our analysis of channel geomorphology necessarily

extends upstream of the pond in order to identify an ideal channel template or

reach. The Project Team also spent time on the project area with TNC personnel

to gather additional data and observations.

Relative to likely historic and pristine conditions we believe that Silver Creek and

at least some reaches of its tributaries are under-potential. The principal

degradation is moderate to excessive siltation compounded by over-wide

channel cross-sections and below potential riparian habitat. This degradation is

manifested in non-equilibrium sediment transport (including the pond being

identified as a sink for gravel), impacts to salmonid habitat including reduction in

suitable spawning substrate, loss of water column depth cover, perhaps unusual

winter turbidity related to aquatic macrophyte senescence and elevation of

stream temperature.

While Silver Creek on the Preserve still supports a robust trout fishery, it is likely

that the impairments are holding it below potential and perhaps exposing it to

future declines related to a variety of negative environmental stressors. There is

no current ability to compare the historic trout populations with current estimates

though the ability to track future changes exists. One area of investigation that

may warrant further exploration relates to the role of aquatic macrophytes in both

creating and limiting habitat and whether these macrophytes are in fact

increasing to a point where optimal trout habitat is being diminished both in the

pond and upstream. This discussion is intrinsically woven into the synergistic

relationship between the stream's sediment load, its role in vegetative

establishment and sediment transport during full florescence and senescence.

The Project Team has concluded that the Kilpatrick Dam and impoundment

impairs the ecological potential of Silver Creek, most significantly by disrupting

natural sediment transport processes and by increasing summer water

temperatures beyond background levels. Importantly, the maximum daily

temperature is reached in the pond at least 6 hours before the rest of the stream,

thereby affecting both the magnitude and duration of adverse temperatures.

Given past examples of downstream fish kills due to high temperature and low

dissolved oxygen content, we consider this to be a periodically significant impact.

Nine alternatives were developed to address identified ecological impairments

ranging from full restoration measures to modest enhancement activities and no

action. The alternatives were then subjected to a preliminary constraints and

benefits analysis and three preferred alternatives were developed in greater

detail along with a cost:benefit analysis. While dam removal is the number one

alternative from an ecological restoration perspective, it was not evaluated in

detail due to the resistance of Picabo Livestock to this alternative. The next

highest alternative is the modification of the dam to add a bottom release outlet

structure (150 cfs capacity). The benefits of such a structure are cool water

release (thermal benefits), hydraulic removal of pond sediments, improved

sediment transport, improved angling by pond deepening, lowering of pond

elevation to allow additional sediment removal and/or revegetation of exposed

sediment. The downstream flushing of sediment would require careful

consideration for the fate of the sediment such that downstream impacts are

mitigated. This alternative would cost on the order of $300,000. Cost:benefit

considered to be good.

The third alternative considered is modification of the dam to lower the outlet

elevation. Lowering the pond elevation would allow access to impounded

sediments for mechanical removal or revegetation. Cost is anticipated to be on

the order of $65,000+. Ecological benefits of this alternative are considered to be

limited. Cost:benefit considered neutral to fair.

The fourth alternative is the active/experimental revegetation of pond sediments

during the non-irrigation season when pond is at low-pool stage. The cost for

this type of effort is anticipated to be on the order of $80,000/acre. The ability for

the planted material to self-propagate after planting and expand coverage of the

sediments is unknown. For this reason we have recommended that if this

alternative is pursued that it be undertaken in a phased manner with observation

of planting success and ability to self-propagate between planting years. We

consider the ecological benefits of this approach to be neutral to potentially fair.

The cost:benefit of this alternative is considered open to discussion.

Two additional discussions are included in this report -- one regarding an

analysis of channel dimension and sediment transport capacity, and a second

discussing the need for a systemic/holistic evaluation of the Silver Creek

watershed trout habitat, riparian vegetation, and geomorphology. The sediment

transport analysis concluded that the channel width:depth ratio of Silver Creek

above and below the pond are not conducive to equilibrium sediment transport.

The discussion of other management alternatives for addressing observed

degradation on the Preserve suggests that TNC should also consider a more

comprehensive aquatic assessment, prioritization and restoration program that

encompasses the Silver Creek tributaries, mainstem and downstream of the

Preserve.

2 INTRODUCTION

Gillilan Associates, Inc. and project partners Applied Geomorphology, Oxbow,

Inc. and Hoitsma Ecological were hired by the Idaho chapter of The Nature

Conservancy to assess historic and current channel conditions on Silver Creek

and the ecological impacts of Kilpatrick Pond and dam, and make

recommendations for addressing identified impacts. Over the course of the

project the Project Team has interacted closely with TNC through conference

calls, on-site meetings, email, and an interim report. We also undertook an

extensive literature review, conducted interviews with key sources of knowledge,

performed an on-site evaluation, and analyzed existing water and channel

geometry data. This report summarizes the results of the investigation,

beginning with a consensus statement of project goals and objectives developed

in conjunction with TNC.

Defined Project Goals and Objectives

Goal : Develop conceptual alternatives and assess feasibility for restoring

or enhancing natural riparian and stream habitat and processes through

Kilpatrick Pond, the approximately one mile reach of Silver Creek spanning both

Picabo Livestock and Nature Conservancy lands and downstream as warranted

by any proposed activity on the project site.

Objectives:

1) Improve overall habitat diversity and ecological function which may

include:

a. Achieving sediment transport continuity

b. Improving habitat connectivity

c. Maximizing riparian potential

d. Reducing thermal impacts of the dam

e. Improving existing fishing opportunities

2) Develop Cost/Benefit assessment for each considered alternative:

a. Provide costs and ecological benefits of selected alternatives from

dam removal to no action.

b. Consider stakeholders needs and views and provide scientific and

professional opinions supporting alternatives.

Summary of Pre-field Activities

Besides review of existing literature we began compiling data relative to the

project on a GIS base map (Appendix A). A principal component of the map is a

stream stationing system that provides a stream centerline distance measured

from the point where Silver Creek crosses Hwy 20 (Station 0+00) so that past,

current and future discussions of the area can use a common reference.

Summary of Field Activities

The Project Team met with TNC onsite January 10-12, 2007 during an unusually

frigid period. Activities undertaken included:

· Visual observation of downstream reaches of Silver Creek (notably Point

of Rocks and upstream).

· Observation of Stalker and Silver Creek via a canoe float from the Stalker

Bridge downstream to Kilpatrick Bridge.

· Meeting with Nick Purdy of Picabo Livestock at the dam site.

· Collection of sediment samples upstream of Kilpatrick Pond.

· Observation of the TNC Stalker Creek enhancement project.

· Observation of Picabo Livestock's enhancement project downstream of

the dam.

· Elevation survey of the Kilpatrick dam and ditch crest elevations.

· Meeting with TNC to vette the first series of project alternatives.

Additional Areas of Investigation

Based on observation of two stream alteration projects undertaken both above

and below the project site that utilized a strategy of channel narrowing to improve

channel dynamics, the Project Team concluded a general analysis of channel

competence (ability to transport sediment at an equilibrium rate) would be helpful

to inform future projects. This analysis will evaluate sediment transport ability

relative to channel slope and cross-section and will point to a theoretical channel

dimension (template).

3 PROJECT SITE HISTORY, DATA AND CONDITION

3.1. Geologic/Geomorphic Setting

Silver Creek and Kilpatrick Pond are located in Blaine County in central Idaho.

The spring creek system is located in the upper Wood River Valley, which is

more broadly located within what is referred to as the Bellevue Triangle (Figure

1). This topographic feature is a triangular shaped valley bounded by the Pioneer

Mountains to the northeast, the Smokey Mountains to the northwest, and the

Picabo Hills to the south. The uppermost part of the Bellevue triangle is near

Hailey, where it is approximately 1 ½ miles wide. South of Bellevue, the valley

widens to over 6 miles in an east-west direction. In this area, a series of river

terraces are collectively known as Poverty Flats, where a drainage divide

separates the Wood River system from that of Silver Creek. Whereas the Wood

River flows to the southwest out of the Bellevue Triangle, Silver Creek flows

south towards the Picabo Hills and then east past Picabo and onto the Snake

River Plain (Figure 1) .

Figure 1. Satellite image of the “Bellevue Triangle”. Silver Creek spring creek system can

be seen draining to southeast. Blue arrows note flow direction.

The Wood River Valley is underlain by Tertiary-age sedimentary rocks. These

units were deposited by the ancestral Big Wood River, as it flowed

southeastward across the valley, entering the valley through a deep canyon north

of Bellevue, and exiting through the gap now occupied by Silver Creek south of

Picabo. In early Pleistocene time, this gap was blocked by basalt flows which

dammed the Big Wood River. The formation of a lake over the valley floor

resulted in deposition of coarse sediments in the northern portion of the valley

where the river entered the lake, and finer sediments on the lake floor to the

south (Moreland, 1977). When the lake filled to an elevation high enough to

overtop a topographic gap on the western margin of the valley, the Big Wood

River relocated to the west. This newly occupied gap was then blocked by

another basalt flow, creating another lake and forcing the river back to the east.

According to Moreland (1977), this process of basalt flows blocking the two exit

points of the valley occurred multiple times, and each time the river deposited

additional alluvium across the valley floor. Synchronous glaciation in the upper

valley resulted in the delivery of large volumes of coarse sediment and runoff,

further accelerating depositional rates in the area. The young, relatively

permeable sediments in the valley coarsen from south to north and reach

thicknesses of up to 500 feet (Moreland, 1977). These sediments are the

primary source of groundwater in the area (Moreland, 1977), with the exception

of the southern portion of the valley, where underlying fractured basalt flows

comprise part of the aquifer system.

The geologic setting of the Bellevue Triangle is critical in understanding the

geomorphology and hydrology of Silver Creek. Silver Creek is a spring fed

system, and its flows are derived from groundwater that is sourced from irrigation

activities, the Big Wood River aquifer, snowmelt, and precipitation. The naturallyderived

water from Big Wood River recharge, snowmelt, and rainfall all provide

water to the Silver Creek hydrologic system; this subsurface water generally

flows southward along a topographic gradient across the Bellevue triangle

towards the Picabo Hills. This groundwater is also augmented by irrigation water

that is diverted eastward from the Big Wood River.

As the groundwater flows southward across the Bellevue Triangle towards the

Picabo Hills, it encounters finer grained, sedimentary units that effectively drive

the water to the surface. Moreland (1977) described these units as “virtually

impermeable” relative to the overlying valley fill. This has resulted in the

formation of a series of spring fed stream systems that typically begin to emerge

about 3 miles north of the Picabo Hills, flowing southward. Near the northern

flank of the Picabo Hills, the streams coalesce into Silver Creek, which flows

eastward along the base of the Picabo Hills. The spring-fed tributaries that

emerge from the valley floor include Stalker, Chaney, Mud, Grove, and Loving

Creeks. As these streams coalesce to form Silver Creek, additional streamflow

is contributed by upwelling along their courses.

3.2. Historic Condition

3.2.1. Pre-settlement Condition

Likely Riparian Condition

Little data is available on the historical condition of Silver Creek riparian zones

though it may be reasonable to infer that a dense, and presumably beaverdominated

riparian zone existed prior to settlement. Reflecting on riparian

conditions after the Oregon Trail era, a 1996 NRCS report noted significant

riparian vegetation degradation and surmised that “the loss of riparian shrubs

was probably a rapid event that culminated shortly after the first homesteaders

and beaver trappers arrived in the area in the late 1800’s and early 1900’s.” The

NRCS also noted that “some local landowners feel that there were never very

many willows or other riparian plants along streams in the Silver Creek

watershed. While this is possible, it is unlikely” (NRCS 1996).

Although there is a great deal of information about instream aquatic vegetation,

almost no information exists about historic riparian vegetation. Fisheries

population surveys have been tied to aquatic plants or channel substrate, with

few links made to riparian cover.

Likely Channel Condition and Planform

The earliest description of the geomorphology of Silver Creek is from the diary of

Winfield Scott Ebey, who recorded observations while traveling though on the

Oregon Trail in 1854. On August 11, 1854, Ebey’s group left the Little Wood

River and drove their wagons “over some low hills (Bradley Summit), to a camp

on Silver Creek. Ebey noted that “This is a beautiful clear stream 25 ft wide, 2 ft

deep and so full of trout that they can hardly swim”. It is not known exactly where

on Silver Creek this 25 ft channel width was estimated so no inference can be

made regarding historic channel top width though it is noted that today’s channel

width commonly exceeds 100 ft in width.

It has been convincingly documented that agricultural land use impacts on Silver

Creek have resulted in increased sediment loading, and hydrologic alteration due

to changes in land cover types as well as irrigation practices. Without presettlement

hydrologic or morphologic data, it is impossible to assess the actual

geomorphic parameters of the stream for that time frame. However, the nature

of these impacts typically include removal of riparian vegetation, bank erosion,

channel widening, and loss of stream sinuosity.

The body of literature on channel changes due to land use is surprisingly absent

with respect to low gradient spring creeks. It is also likely that there are few if

any "pristine" low-gradient, broad valley spring creeks in the region that could be

utilized for functional reference points. However, our professional observation of

spring creek systems in the Intermountain West suggests the following:

· In a pristine condition, (barring the effects of beaver dams), it can be

assumed that channel width:depth relationships were created and

maintained based on a balanced (equilibrium) sediment transport regime.

· Spring creek banks are particularly susceptible to obliteration through

livestock trampling due in part to their typically fine-grained and frequently

saturated soils. Repeated bank trampling leads to channel widening.

· Once widened, spring creeks display little propensity for natural processdriven

re-narrowing even with a change in land management. This is

largely associated with the absence of a hydrologic regime capable of

significantly re-sorting in-channel sediments into lateral and point bars that

become revegetated and form new channel banks.

· Once channel competence is compromised by channel over-widening,

and combined with continued input of sediment into the stream, channel

cross-section complexity decreases and frequently become plane bed with

scour features limited to planform elements (outside bends) or in-channel

roughness (roots, woody debris, emergent vegetation).

It is plausible that Silver Creek had a much narrower channel in non-beaver

ponded reaches and that the current channel dimension has a wider cross

section and straighter planform.

For the purposes of this project, postulated historic dimensions may not be useful

in determining geomorphic restoration parameters for the channel. As the

morphology of the stream is largely governed by sediment load, hydrology,

vegetation, and slope, it is important to consider those parameters under current

management practices. For restoration planning, appropriate channel geometry

can be developed based on an assessment of existing sediment transport

conditions within the preserve (Appendix B).

3.2.2. Post-settlement Changes to Project Reach

Agricultural Practices

Historic changes to Silver Creek following European settlement are considered to

have degraded the geomorphic condition of the channel (Perrigo, 2006). Of

primary concern is a trend of excess sedimentation in the spring creek system

due to increased sediment loads and decreased transport competency.

Prior to the introduction of agriculture, the Silver Creek area was in a relatively

pristine state (Perrigo,2006). With the onset of agriculture it is known that the

natural vegetation was affected, and banks were destabilized due to grazing in

the riparian zones. Some riparian clearing occurred in the early 1900’s in an

effort to increase land area available for agriculture. The effects of these impacts

would have been increased sediment loads and runoff, as well as reduced bank

stability.

Manuel and others (1979) noted that land use changes that occurred around

1950 resulted in an increase in fine sediment loading to Silver Creek. Increases

in fine sediment loading was likely in part due to several projects aimed at

increasing drainage in the area for agricultural purposes. These projects include

the Patton Drain (1952) which included channelization in the upper Stalker Creek

drainage, Patterson Drain (1945), and Daly Ditch (1955), which augmented flows

in Loving Creek. Manuel (1979) also referred to at least one time frame, the

spring of 1974, when persistent winds during a dry spell resulted in “intolerable

deposition” in Loving Creek, prompting the construction of a sediment basin

upstream of the fish hatchery. This study concluded that in the late 1970’s, the

largest source of sediment to Silver Creek was fluvial transport from contributing

tributaries. The results of that study indicated that Stalker Creek contributed 62%

of the sediment in Silver Creek, whereas Grove Creek and Loving Creek

contributed 23% and 15% respectively. Fairly coarse sediment (<4mm)

dominated the bed on Silver Creek upstream of Sullivan Slough, but

downstream, 75% of the bed material was less than 0.25 mm in diameter.

Manuel measured the thickness of fine sediment accumulations and found that it

ranged from 5 cm thick near Grove Creek to over 17 cm downstream of Loving

Creek.

Manuel (1979) reported that where the Lower Patton Drain joined Stalker Creek,

the water from the drain had much higher sediment concentrations than the

creek (78 mg/l vs 22 mg/l). Furthermore, “large piles of dirt line the banks along

the drain and constitute potential sources of sediment”. Manuel concluded that

“It is reasonable to assume that the combined effects of channelization and

straightening of the stream, the network of drainage canals, and the extensive

areas of exposed soil have resulted in the addition of significant amounts of silt to

Stalker Creek. The hydrological regime of Stalker Creek has apparently been

modified by the rapid draining of the adjacent fields, resulting in higher discharge

rates, and for a brief period of time increases in the erosional and sediment

carrying capacity of the stream”.

Since the mid-1970’s, management activities within the area have included

native vegetation restoration, riparian fencing, and bioengineered bank

stabilization. These efforts have improved the density of riparian vegetative

cover and improved the integrity of the streambanks (Perrigo 2006). Flood

irrigation practices have been replaced by sprinkler irrigation typically reduces

sediment loading from fields.