Simulation of Flow and Habitat Conditions Under Ice, 

Cache la Poudre River — January 2006 

By Terry Waddle 

Open-File Report 2007–1282 

U.S. Department of the Interior 

U.S. Geological Survey 
U.S. Department of the Interior 

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey 

Mark D. Myers, Director 

U.S. Geological Survey, Reston, Virginia 2007 

For product and ordering information: 

World Wide Web: http://www.usgs.gov/pubprod 

Telephone: 1-888-ASK-USGS 

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its natural and living resources, natural hazards, and the environment: 

World Wide Web: http://www.usgs.gov 

Telephone: 1-888-ASK-USGS 

Suggested citation: 

Waddle, T.J., 2007, Simulation of flow and habitat conditions under ice, Cache la Poudre River— 

January 2006: U.S. Geological Survey Open-File Report 2007–1282, 37 p. 

Any use of trade, firm, or product names is for descriptive purposes only and does not 

imply endorsement by the U.S. Government 

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report. 

ii
Contents 

Introduction .........................................................................................................................................................................1

Methods ...............................................................................................................................................................................3

Field Data Collection ......................................................................................................................................................3

Ice Cover Construction ..................................................................................................................................................4

Hydrodynamic Modeling ...............................................................................................................................................6

The River2D Model .....................................................................................................................................................6

Basic Governing Equations ...................................................................................................................................6

Incorporating Ice in River2D.....................................................................................................................................7

Habitat Model..................................................................................................................................................................8

Habitat Suitability Criteria .............................................................................................................................................9

Results ................................................................................................................................................................................10

Ice Cover Conditions ....................................................................................................................................................10

Depth and Velocity Under Ice ....................................................................................................................................13

River2D Performance With Ice Cover.......................................................................................................................16

Effects of Ice Cover on Flow Characteristics ..........................................................................................................17

Physical Effects of Winter Release ...........................................................................................................................18

Winter Habitat ...............................................................................................................................................................20

Discussion..........................................................................................................................................................................23

Products of this Study......................................................................................................................................................25

References Cited ..............................................................................................................................................................25

Appendix 1. Habitat Suitability Criteria from Thomas and Bovee (1993) ................................................................27

Appendix 1. Concluded....................................................................................................................................................28

Appendix 2. Discharge and Ice Thickness Measurements November 2005–February 2006, 

Transcribed Field Notes ..................................................................................................................................................29

Appendix 3. Ice Thickness Measurements January 24, 2006...................................................................................33

Figures 

1. Location of Cache la Poudre River study site ...........................................................................................................2

2. Diagram of Cache la Poudre River headwaters showing relationship between headwater 

reservoirs and Kinikinik study site...................................................................................................................................3

3. Study site schematic showing locations of measured cross sections and distribution of ice cover 

based on 2.83 m3

/s conveyance area and local gradient............................................................................................6

4. Discharge transect observed December 15, 2005, WSL = water surface elevation........................................11

5. Discharge transect observed January 24, 2006, WSL = water surface elevation............................................11

6. Pool transect showing average ice thickness of 0.6 m, edge ice thickness determined by 

extrapolation for illustration purposes .........................................................................................................................12

7. Mean daily air and water/ice temperature observed at the Kinikinik study site November 22, 2005 

– April 14, 2006...................................................................................................................................................................13

iii
8. Observed and simulated velocity profile obtained for discharge of 0.608 m3

/s observed January 24, 

2006 .....................................................................................................................................................................................14

9. Depth distribution under ice cover for discharge of 0.608 m3

/s, January 24, 2006 ...........................................15

10. Velocity distribution under ice cover for discharge of 0.608 m3

/s, January 24, 2006; Qin = inflow 

discharge, Qout = simulated outflow discharge.........................................................................................................16

11. Area occupied by depth increments with and without ice at 0.566 m3/s; D = depth ......................................17

12. Area occupied by velocity increments with and without ice at 0.566 m3

/s; V = velocity...............................18

13. Area occupied by depth increments at 0.608 m3/s and 0.325 m3/s.....................................................................19

14. Area occupied by velocity increments at 0.608 m3/s and 0.325 m3/s .................................................................20

15. Habitat area under ice for V<0.15 m/s ....................................................................................................................22

16. Pool areas at 0.608 m3/s using the Forest Service criteria; gray lines are topographic contours...............23

Tables 

1. Pool patch criteria binary pairs .................................................................................................................................10

2. Habitat area (Weighted Usable Area) for two discharges at the Kinikinik site for ice conditions 

equivalent to those observed January 24, 2006 and for open channel conditions ..............................................21

1-1. Brown Trout adults active. (Data from South Platte River, Colorado.) ...........................................................27

1-2. Rainbow Trout adults active. (Data from South Platte River, Colorado.)........................................................28

2-1. Discharge measurement November 22, 2005 ......................................................................................................29

2-2. Discharge measurement December 12, 2005 ......................................................................................................30

2-3. Discharge measurement January 24, 2005 ..........................................................................................................31

2-4. Discharge measurement February 23, 2005.........................................................................................................32

3-1. Cross Section 1 from survey grade Global Positioning System (GPS) ............................................................33

3-2. Cross Section 1 from tag line..................................................................................................................................34

3-3. Cross Section 2 from survey grade Global Positioning System (GPS) ............................................................35

3-4. Cross Section 3 from survey grade Global Positioning System (GPS) ............................................................36

3-5. Cross Section 4 from survey grade Global Positioning System (GPS) ............................................................37

iv
v

Conversion Factors 

Inch/Pound to SI 

Multiply By To obtain 

foot (ft) 0.3048 meter (m) 

square foot (ft2

) 0.0929 square meter (m2) 

cubic foot (ft3

) 0.02832 cubic meter (m3) 

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s) 

SI to Inch/Pound 

Multiply By To obtain 

meter (m) 3.281 foot (ft) 

square meter (m2

) 10.76 square foot (ft2) 

cubic meter (m3

) 35.31 cubic foot (ft3) 

cubic meter per second (m3

/s) 35.31 cubic foot per second (ft3/s) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: 

°F=(1.8×°C)+32 

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: 

°C=(°F-32)/1.8 

Vertical coordinate information is referenced to World GeodedicNorth American Vertical Datum of 1988 (NAVD 88).” 

Horizontal coordinate information is referenced to the insert datum name (and abbreviation) here for instance, “North 

American Datum of 1983 (NAD 83).” 

Altitude, as used in this report, refers to distance above the vertical datum. 
Simulation of Flow and Habitat Conditions Under Ice, 

Cache la Poudre River — January 2006 

By Terry Waddle1

Introduction 

The U.S. Forest Service authorizes the occupancy and use of Forest Service lands by 

various projects, including water storage facilities, under the Federal Land Policy and Management 

Act. Federal Land Policy and Management Act permits can be renewed at the end of their term. 

The U.S. Forest Service analyzes the environmental effects for the initial issuance or renewal of a 

permit and the terms and conditions (for example, mitigations plans) contained in the permit for the 

facilities. The U.S. Forest Service is preparing an environmental impact statement (EIS) to 

determine the conditions for the occupancy and use for Long Draw Reservoir on National Forest 

System administered lands. The scope of the EIS includes evaluating current operations and effects 

to fish habitat of an ongoing winter release of 0.283 m3

/s (10 ft3/s) from headwater reservoirs as part 

of a previously issued permit. The field conditions observed during this study included this release. 

The U.S. Forest Service entered into an interagency agreement (05-IA-11021000-030) with 

the U.S. Geological Survey (USGS) Fort Collins Science Center to perform analysis of fish habitat 

and flow relationships in the Cache la Poudre River during winter ice-over conditions using a twodimensional hydrodynamic model. The U.S. Forest Service selected the Fort Collins Science 

Center for this task because of their expertise in developing two-dimensional hydraulic models for 

habitat modeling applications. This report transmits model results to the U.S. Forest Service to 

analyze the effects of alternative flow scenarios at a site on the mainstem Cache la Poudre River in 

Larimer County, Colorado, near Kinikinik (40° 42' 44.16" N. lat, 105° 44' 30.70" W. log), as shown 

in figure 1. It will be used in pending environmental analyses and decisions for the occupancy and 

use of the Arapaho-Roosevelt National Forest by water storage facilities. 

The water management scenarios of interest in this study are related to releasing water from 

Chambers and Barnes Meadows Reservoirs, based on the assumption that winter flow 

augmentation can increase potential fish habitat. Figure 2 shows the relationship between 

Chambers, Barnes Meadows, and Long Draw Reservoirs. At the time this study was proposed, 

existing flow simulation results showed that the channel constraints imposed by existing artificial 

low-head dikes would have little or no effect on the hydrodynamics of the river at the low flow 

levels that were to be evaluated. The Kinikinik study site contains deep pools, riffles, and runs. This 

diversity of habitat types made it ideal for assessing the effects of altered flow on fish habitat under 

ice in the main stem Cache la Poudre River. Thus, the Kinikinik site was selected for this study of 

winter habitat conditions. 

 

1

 Hydrologist, U.S. Geological Survey, Fort Collins Science Center, 2150 Centre Ave., Fort Collins, CO, 80526 

1
The preexisting topographic and hydrologic data collected at this site enabled data 

collection efforts for this study to focus on describing streamflow and ice cover during the winter 

months. A two-dimensional hydrodynamic model, River2D (Steffler and Blackburn, 2002), was 

used to simulate flow conditions under the ice cover that was observed January 24, 2006. 

The objectives of this study are (1) to describe the extent and thickness of ice cover, (2) 

simulate depth and velocity under ice at the study site for observed and reduced flows, and (3) to 

quantify fish habitat in this portion of the mainstem Cache la Poudre River for the current winter 

release schedule as well as for similar conditions without the 0.283 m3

/s winter release. 

Study Site Location

Cache la Poudre River

Figure 1. Location of Cache la Poudre River study site. 

2
Figure 2. Diagram of Cache la Poudre River headwaters showing relationship between headwater 

reservoirs and Kinikinik study site. 

Methods 

An existing river flow and habitat model was applied to simulate flow and habitat 

conditions under ice. Model input requirements dictated the data collection, formatting, and 

modeling steps described below. 

Field Data Collection 

Two-dimensional models of open channel hydrodynamics require that a specific set of 

boundary conditions be supplied to the model. Among those boundary conditions are a threedimensional digital elevation model, an outflow water surface elevation, and an inflow discharge 

rate. Data must be obtained from the study site for each of these required conditions. 

Three-dimensional topographic data were obtained for the Kinikinik site using Trimble 

4800 and 5800 survey grade global positioning system (GPS) receivers employing real time 

kinematic position recording and multi-path reduction (Trimble Navigation, Ltd., 

http://www.trimble.com). Both systems use carrier phase processing that enables centimeter 

accuracy in the field. Certain areas of the study site were occluded from GPS satellite reception by 

surrounding topography and trees. Survey of the occluded areas was conducted using a Leica 3-

3
second precision total station. All data were collected in Universal Transverse Mercator (UTM) 

coordinates (zone 13 north, datum WGS 1984). 

A total of 3,426 topographic observations were obtained with the GPS and total station 

equipment from 2003 to 2006. A triangulated irregular network (TIN) was applied to the data to 

construct a digital elevation model of the study site. An additional 1,581 points were added to the 

topographic data set using linear interpolation to remove triangulation anomalies. The channel 

appears to have been quite stable from 2003 to the present, so the observed channel configuration 

was used without collection of additional topographic data. 

Air and water temperature and pressure were recorded from November 22, 2005 to April 

14, 2006, using Solinst Levelogger (http://www.solinst.com) recording pressure transducers. The 

Solinst system is capable of providing a continuous depth-of-flow record at a deployed location 

when both air and water pressure are recorded to allow correction for atmospheric conditions. The 

transducer deployed in the stream became encased in ice and did not yield a meaningful indication 

in changes in depth during this period, but both air temperature and temperature within the ice were 

recorded. 

Because the focus of this study is to describe flow under ice cover, free-surface water data 

collected when the channel was ice free were not applicable. During the winter of 2005–2006, 

discharge and ice cover data were collected in cooperation with U.S. Forest Service personnel. The 

conditions on January 24, 2006, were the most extensively sampled and are the conditions 

represented in this study. 

On January 24, 2006, a discharge measurement (0.608 m3

/s, 21.43 ft3/s) was obtained at a 

steep gradient section where there was an open lead (an area that was not completely covered by 

ice). In the portions of the discharge measurement cross section that were covered by ice, holes 

were drilled through the ice and a Marsh McBirney flow meter attached to a standard topset wading 

rod was inserted. Velocity observations were obtained at 0.2 and 0.8 times the depth in the areas 

covered by ice and at 0.6 times the depth in the open areas. The depth at all velocity measurement 

points did not exceed 0.2 m (1.7 ft) so it was deemed adequate to make the measurements at 0.6 

times the depth in the open lead. 

Ice Cover Construction 

On January 24, 2006, ice thickness data were obtained for four cross sections including the 

discharge transect. The data were obtained by drilling through the ice and measuring thickness 

using a 0.015 m square stick ruled in both meters and feet with a 0.05 m angle bracket attached at 

the zero end of the stick. This allowed the measuring stick to be inserted into holes in the ice and 

hooked on the bottom of the ice, giving a direct reading of ice thickness at the surface. Depth from 

the ice surface to the bed was measured using the measuring stick or a standard level rod extended 

as necessary to reach the bottom in deep pools. Two cross sections were placed at known areas of 

high gradient, one in a pool and one in a transition from narrow channel to pool. Due to the 

thickness and hardness of the ice, a total of 47 observations were obtained. Each ice thickness 

observation was located using the GPS equipment by measuring XY coordinates, and ice surface 

elevation. Locations of ice and discharge measurement cross sections are shown in figure 3. See 

appendix 3 for the measured data. 

The observed edge of the ice cover was found to correspond well to the water’s edge 

derived from a simulated free-surface discharge of 2.83 m3/s (100 ft3/s). Thus the extent of ice 

surface for the entire study site was defined using the inundated area obtained from the 2.83 m3/s 

free surface simulation. 

4
The ice thickness cross section data indicated that ice froze to a uniform depth of 

approximately 60 cm in quiescent pools but formed a “flow tube” in areas of higher gradient. This 

“flow tube” effect appears to represent an equilibrium between thermal forces acting to solidify the 

water and the capacity of the stream to transport water at a sufficiently high velocity that ice 

crystals do not attach to the ice cover. Thus, lower velocity edges tended to freeze to the bed, 

forcing the flow toward the center of the channel. 

Ice thickness was distributed over the entire domain of the study site by constructing similar 

cross sectional flow tubes in areas with gradients similar to the measured cross sections and 

interpolating between those approximated cross sections. A total of 75 cross sections were 

developed beginning with an upper ice surface derived from the 2.83 m3/s flow simulation and 

adjusting ice thickness to produce cross sectional profiles that yielded discharge conveyance areas 

similar to those observed. The resulting ice cover is shown in figure 3. The gray lines indicate 

simulated 0.2 m ice thickness contours and the blue line indicates the simulated water’s edge. 

The average ice thickness observed over the pools was 0.60 m and was found to be 

consistent across both measured pools. Thus the ice thickness over all pools was set to 0.60 m or 

the depth from the estimated ice surface to the bed and adjusted where there was insufficient 

conveyance area between the bottom of the ice and the bed. Areas of steeper gradient required 

slightly less cross sectional area and areas of milder gradient required larger cross sectional area to 

convey the discharge. By contouring the conveyance areas to match observed areas, the estimated 

ice cover is believed to closely approximate the ice formation occurring over the unmeasured 

portions of the study site. Figure 3 illustrates the derived ice thickness and shows the areas 

measured on January 24, 2006. 

5
Figure 3. Study site schematic showing locations of measured cross sections and distribution of ice 

cover based on 2.83 m3

/s conveyance area and local gradient. 

Hydrodynamic Modeling 

The River2D model (Steffler and Blackburn, 2002) was selected for this study because of 

its capability to represent hydrodynamic conditions under ice cover. Numerous other twodimensional hydrodynamic models are available; however, the ability to represent displacement 

and flotation of an ice cover is a unique feature of the model. 

The River2D Model 

The River2D model uses the finite element method to solve the “shallow water” equations. 

The following description of River2D is adapted from Steffler and Blackburn (2002). 

Basic Governing Equations 

The basic equations of two-dimensional models describe mass and momentum conservation 

in two dimensions. In River 2D, the differential equation of mass continuity is represented as, 

6
( ) ( ) = 0

∂

∂

+

∂

∂

+

∂

∂

y

HV

x

HU

t

H , (1) 

x and y

directio

The conservation of x momentum equation is rep

where: H is the depth of water, U and V are the velocity components in the 

ns, respectively, and t is time. 

resented as, 

( ) ( )

( ) ( ) ( )⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

∂

∂ ⎟ +

⎠

⎞ ⎜

⎝

⎛

∂

∂

fxx +−= xx H xy τ

ρ y

Hτ

ρ x

SSgH 1 1

0

where: S = bed

∂

∂

+

∂

∂

+

∂

∂

+

∂

∂ x x

x H

x

g Vq

y

Uq

xt

q

2

2

 (2) 

0x slope in the x direction, Sfx = τbx/(ρgH) is the friction slope in x, τbx is the bed shear 

in x, ρ is density and g is the gravitational constant. A similar equation describes the y component 

of momentum: 

( ) ( )

( ) ( ) ( )⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

∂

∂ ⎟ +

⎠

⎞ ⎜

⎝

⎛

∂

∂

fyy +−= yx H yy τ

ρ y

Hτ

ρ x

SSgH 1 1

0

Relations for the bed and side shear stresses must be specified. Steffler and Blackburn 

(2002) point out (p 11): “Since these st

∂

∂

+

∂

∂

+

∂

∂

+

∂

∂

y y

y H

x

g Vq

y

Uq

xt

q

2

2

 . (3) 

resses arise primarily from turbulent flow interactions, there 

is considerable uncertainty i

equation is used for the friction slope, 

n their evaluation. Typically, a two-dimensional form of Manning's 

34

2 22

/ fx H

+VUUn S = , (4) 

and a Bousinessq type eddy viscosity is used for the transverse shear, 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

∂

∂

+

∂

∂ = xV

y

U

ντ txy .” (5) 

They further explain: “The parameters n and νt are not constants or fluid properties, but 

depend on the flow situation. As a result they become the “tuning” or calibration parameters that 

may be changed to bring a model prediction into agreement with measured data.” (Steffler and 

Blackburn, 2002, p 11) Note: νt

 is commonly called the eddy viscosity and n is usually referred to 

as Manning’s n. 

Due to the interdependence of terms, these coupled differential equations cannot be brought 

to a closed form solution using the tools of algebra and differential calculus. Thus, models that 

okes equations must rely on numerical techniques that are typically 

ers. 

solve these or the full Navier-St

implemented on digital comput

Incorporating Ice in River2D 

River2D is equipped to model flow under a floating ice cover with known geometry. The 

ice thickness and roughness must be defined over the entire solution domain. When ice cover is 

present on a river, it affects the flow hydraulics in a number of ways: (1) roughness and shear stress

operate on both the bed and the bottom of the ice cover, (2) velocity is redistributed due to the 

thickness of the ice cover, (3) average velocity may be reduced due to greater overall resistance or 

7
increased due to constraining the conveyance area of the flow, and (4) the calculated water surface

elevation is increased to accommodate the submerged thickness of the ice cover. These conditions 

are accommodated in the River2D model by modifications to the x and y momentum equations to 

account for submerged depth of the ice cover (x equations shown here) 

( ) ( )

( ) ( ) ( )⎟

⎟ ⎝ ∂ ⎠ ⎝ ∂ ⎠ fxx xx xy ρ x ρ y

0

⎞

⎜

⎜

⎛ ∂ ⎟ +

⎞ ⎜⎛ ∂

+−=

∂

∂ −⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

∂

∂

+

∂

∂

+

∂

∂

+

∂

∂ s x x

x

SSgD Dτ Dτ

x

H

gt

H

x

gVq

y

Uq

xt

q

1 1

2

2

and by developing a combined ice and bed resistance model using an average shear stress 

2

ixbx

fx

ττ

τ

+ = . 

Then the friction slope incorporating becomes ice

U

gDC

VU

ρgD

τ

S s

fx

fx 2

2 2 + == . 

In these equations, ts

 is the submerged portion of the ice cover, D is the depth of flow from the bed 

to the bottom of the ice cover, H is the depth to the free surface, τfx is the average shear stress in x, 

τbx is the bed shear stress, τix is the shear stress due

22

 to friction with the bottom of the ice cover, S is 

the resu

red 

e (no ice) model and then adding ice cover and re-solving for the ice cover 

condition. Calibration and verification of the study site is required as it is with any free surface 

ver2D. 

s application: a spatially explicit form of the 

Weight atch 

tat 

unding a point, multiplies that 

composite scalar times each tri utary area and sums the result over the study site domain. The most 

commonly used form is a linear product that can be represented as: 

sci is 

composite suitability as a geometric mean of the suitability product or select the minimum 

fx

lting friction slope in x, and Cs

 is the Chezy coefficient. See Steffler and Blackburn, 2002, 

for full development of the ice resistance model. 

Execution of the River2D model with ice cover is accomplished by first running the desi

discharge as a free surfac

application of Ri

Habitat Model 

Two habitat model concepts were used in thi

ed Usable Area (WUA) model (Bovee, 1982; Bovee and others, 1998) and a habitat p

model such as used by Bowen, and others, (2003). 

The fish habitat component contained in River2D is an implementation of the WUA 

concept similar to that used in the PHABSIM (Milhous, and others, 1989) family of riverine habi

models. WUA is calculated as an aggregate of individual habitat suitability indices (HSI, range 0.0 

- 1.0) for depth, velocity and a channel descriptor that is evaluated at every point in the domain. 

WUA can be calculated by any method that integrates the individual HSI into a composite scaling 

value (composite suitability index, CSI) for each tributary area surro

b

= ∑

i

*a*s*ssWUA icidv

where: sv

 is the habitat suitability index for velocity, sd is the habitat suitability index for depth, 

the habitat suitability index for channel index, and ai

 is the tributary area to node i. Here, the

product of the three suitability indices is the CSI. Other forms of the WUA calculation use the 

8
suitability value as the scalar. In River2D, the nodes noted above are the computational nodes of

the finite element mesh, and the tributary areas are the "Thiessen polygons", including the area 

closer t

f 

ice to select the most 

ppropriate metric for defining trout habitat in the presence of ice. 

Habitat Suitability Criteria 

riteria 

r temperatures near 0°

 C. The habitat suitability criteria used in 

this stu

ly removes channel index from the 

WUA c

in 

th 

o a particular node than all other nodes. 

The habitat patch model used in this application is designed to select the area of the study 

site that classifies as pool habitat using various quantitative definitions of a pool. Here, pools are 

defined using binary criteria and calculated as the aggregate of all areas having (1) a depth greater

than the defined threshold and (2) an average velocity less than the defined threshold. A range o

binary threshold values are applied and reported to enable the Forest Serv

a

In the Weighted Usable Area concept, a set of habitat suitability indices (one index scale 

each for depth, velocity, and channel index) for a particular organism is often referred to as a set of 

habitat suitability criteria or simply habitat suitability criteria (HSC). The habitat suitability c

used in this study are taken from Thomas and Bovee (1993). These criteria were developed 

independently from this investigation and were tested for transferability to the Cache la Poudre

River. The habitat suitability criteria from Thomas and Bovee (1993) used herein are for adult 

rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) in open channel conditions 

affected neither by ice nor low wate

dy are given in appendix 1. 

The Thomas and Bovee (1993) study characterized channel index using a cover code based

on combinations of visual isolation and velocity shelter. Such cover data were not collected at the 

Kinikinik study site due to ice cover. Hence, to facilitate calculation of habitat areas based on the 

Thomas and Bovee depth and velocity criteria, all channel index nodal values were set such that the 

resulting suitability index would be 1.0. This approach effective

alculation and focuses the results on depth and velocity. 

The following general observations regarding over-winter habitat use and behavior are 

taken from Lindstrom and Hubert (2004), Simpkins and others (2000), Jakober, and others 1998, 

Cunjak 1996, Chisholm, and others 1985. One or more of these authors reported (1) fish locate 

deep pools with low velocity areas to conserve energy during the winter, (2) fish actively feed 

during winter, (3) fish have strong site fidelity to a relatively small area, and (4) that deep slow 

areas afford protection from frazil ice events. Lindstrom and Hubert (2004) and Simpkins and 

others (2000) found trout in deep pools with low velocities and in stable areas during the winter 

under ice and in near freezing conditions. Simpkins and others (2000) found that wild trout in the 

Bighorn River below Boysen Dam moved to deeper (>1.49 m) and slower water (<30 cm/s) with 

the onset of winter. The preferred areas occurred near the bottom of the stream in deep pools wi

adjacent water velocities greater than 15 cm/s. Lindstrom and Hubert (2004) noted site fidelity 

during the winter because few fish moved once they had located in deeper areas of the stream. 

Simpkins and others (2000) reported, trout remain active in winter but they are not as active as 

during the other seasons. Harper and Farag (2004) reported disproportional use of off-channel 

pools, runs, and riffles. Yellowstone cutthroat trout (O. clarki bouvieri) were found in mu

proportions in pools and runs than the available proportion of those habitats and in lower 

proportions than available riffle habitat. Harper and Farag (2004) reported that only 3 percent of t

observed Yellowstone cutthroat trout used riffles, yet riffles constituted from 40 to 60 percent 

available habitat. The Yellowstone cutthroat trout were frequently found in deep run habitat. 

ch higher 

he 

of 

Overall, these observations suggest that habitat suitability criteria describing winter habitat use 

9
would rank deep pools with low velocities as better habitat than shallow pools with higher 

velocities. 

The Forest Service provided a set of binary hydraulic criteria to the USGS to perform a 

habitat patch analysis of the amount of deep slow water. (R. Deibel, pers. comm.) The criteria 

require depths greater than 1 meter and velocities less than 0.3 m/s. Thus the patch-based habitat 

analysis during the winter focuses on identifying main channel pools and is based on assumed 

behavior of trout under ice such as cited above (R. Deibel, personal communication, 2007). There 

will be some fish that use microhabitats that may differ from the above criteria. The Forest Service 

criteria of depth >1 meter and velocity <0.3 m/s could also cause the model to select deep runs in 

the Kinikinik study site. 

In addition to the binary pool patch criteria supplied by the Forest Service further 

combinations of minimum depth and maximum velocity were evaluated to better define the range 

of characteristics of the study site in winter. Each pair of pool limiting criteria was applied to the 

entire site for the 0.608 m3

/s and 0.325 m3/s discharges to obtain the area of pool habitat derived 

from those limits. Table 1 summarizes the ten patch criteria threshold pairs evaluated: 

Table 1. Pool patch criteria binary pairs. 

Minimum 

depth (m) 

Maximum 

velocity (m/s) 

1.5 0.3 

1.5 0.15 

1.0 0.3 

1.0 0.15 

0.75 0.3 

0.75 0.15 

0.5 0.3 

0.5 0.15 

0.3 0.3 

0.3 0.15 

Results 

Ice Cover Conditions 

As noted above, ice thickness over low velocity areas and pools was found average 0.60 m. 

A thermal regime capable of producing this much ice over a pool can cause ice at the edge of the 

flowing stream to freeze to the bed. In areas of steeper gradient, the result is constriction of the 

flowing water into a tube-like cross section as shown in figures 4 and 5. 

10
VIEW OF TRANSECT FACING DOWNSTREAM

2,355.6

2,355.7

2,355.8

2,355.9

2,356.0

2,356.1

2,356.2

2,356.3

01234567

STATION, IN METERS

ELEVATION, IN METERS

TOF OF ICE

BOTTOM OF ICE

BED

WSL

Figure 4. Discharge transect observed December 15, 2005, WSL = water surface elevation. 

VIEW OF DISCHARGE TRANSECT FACING DOWNSTREAM

2,355.4

2,355.5

2,355.6

2,355.7

2,355.8

2,355.9

2,356.0

2,356.1

2,356.2

2,356.3

8 9 10 11 12 13 14 15

STATION, IN METERS

ELEVATION, IN METER

TOP OF ICE

BOTTOM OF ICE

BED

WSL

Figure 5. Discharge transect observed January 24, 2006, WSL = water surface elevation. 

The cross sections in figures 4 and 5 were located within 5 m of each other in a relatively 

steep gradient section of the site. They were selected for discharge measurement because the open 

lead allowed the ice thickness near the edge to be readily observed so field crew members could 

move about with greater safety than in an area of unknown thickness. The location of the open lead 

in this portion of the channel had shifted between the two dates. Note that in both cases ice had 

frozen to the bed at the sides of the channel to thicknesses ranging from 0.3 to 0.6 m, confining the 

flow to the center of the channel. Midchannel velocities ranged from 0.6 to 1.0 m/s. Appendix 2 

contains the observed ice and flow data that are summarized here. 

In pools, the ice covered the water surface in a more uniform manner as shown in figure 6. 

The variation in measured thickness of the ice cover may be due to the ice breaking away when the 

digging bar broke through. 

11
VIEW OF POOL TRANSECT FACING DOWNSTREAM

2,354.0

2,354.5

2,355.0

2,355.5

2,356.0

2,356.5

2,357.0

2,357.5

2,358.0

0 5 10 15 20 25

STATION, IN METERS

ELEVATION, IN METERS

TOP OF ICE

BOTTOM OF ICE

BED

Figure 6. Pool transect showing average ice thickness of 0.6 m, edge ice thickness determined by 

extrapolation for illustration purposes. 

Comparison of the bed elevations in the two deepest pools determined during open water 

surveys and those determined by measuring through the ice revealed differences as great as 0.3 m 

with the winter measurements resulting in lower bed elevations in those pools. Several factors may 

have influenced this result, including (1) measuring at near but different, XY locations; (2) 

penetration of softer bed sediments in winter, because the field crew could establish firmer footing 

on the ice than they could raft in summer, and possible scour or deposition of sediment in the 

bottom of the pools between measurements. Low flow dynamics in the pools are not significantly 

affected by these elevation differences. The pool bed elevation differences were assumed to have 

negligible effect on simulated habitat values. 

Figure 3 presents the ice cover map developed for these simulations. To ensure that all flow 

margins produced realistic edge wetting, a 0.05 m ice thickness was assigned to all areas above the 

simulated water surface for the target discharge. Thus, the minimum thickness encountered over 

dry portions of the study site is 0.05 m. The ice cover is reduced to a zero thickness in the areas 

where narrow open leads (areas with no ice cover) were observed on January 24, 2006. This 

convention conforms to the observed increase in surface elevation of the ice at the stream edge 

obtained from the GPS measurements of January 24, 2006. 

The recorded temperatures and pressures indicated that the ice cover broke up by March 29, 

2006, because diurnal fluctuations of water temperature and pressure resumed on that day. Realistic 

depth values were reported from November 22 to November 29, 2005, after which ice influence or 

complete encasement occurred. Realistic depths were again recorded beginning March 29, 2006. 

Temperatures recorded by the ice-encased instrument did not exceed 0°C from December 04, 2005 

– March 29, 2006 with significant periods substantially below 0°C from December 5 to December 

23, 2005 and January 13 – March 5, 2006. The mean daily air temperature data (see fig. 7) show a 

significant cold period from December 2 to December 21, 2005, a warmer period from December 

21, 2005 to January 15, 2006, and another cold period from January 16 to February 26, 2006. 

Measured discharge transects such as shown in Figures 4 and 5 do not show significant changes in 

the ice cover thickness between measurements. Lack of significant change in the measured 

transects suggests the warmer period was not sufficiently warm to change the ice cover. This 

assertion is further supported by the temperature recorded by the ice-encased transducer placed in 

12
the stream (fig. 7). Note that the temperature within the ice dropped significantly in response to 

periods of extremely cold air. The ice buffered the temperature variation of the instream transducer, 

but the extremely cold periods are likely to represent times when the erosion of ice from the bottom 

of the ice sheet was balanced by freezing from the sides. The ice cover observed on January 24, 

2006 appears to have formed during the December cold period and was retained until at least midMarch. Thus, relatively stable channel and hydraulic conditions occurred from late November until 

ice break up after mid-March. 

MEAN DAILY TEMPERATURE

-25

-20

-15

-10

-5

0

5

10

15

11/21/05 12/11/05 12/31/05 1/20/06 2/9/06 3/1/06 3/21/06 4/10/06

AIR TEMPERATURE °

C

-2

0

2

4

6

8

10

12

14

WATER/ICE TEMP. °

C

AIR WATER/ICE

Figure 7. Mean daily air and water/ice temperature observed at the Kinikinik study site November 

22, 2005 – April 14, 2006. 

Depth and Velocity Under Ice 

Due to the substantial difficulty encountered drilling large holes through the ice, velocity 

measurements were obtained for the discharge cross section only. Larger holes were required for 

insertion of velocity measuring apparatus than were required for measuring ice thickness. Due to 

this limitation, River2D was relied on to provide depth and velocity values for most of the study 

site. Depth and velocity values at the locations of the discharge transect, and depth and ice 

thickness at the other measured transects were compared with model results to verify model 

performance under the ice cover. 

The empirical observations show surprisingly high velocity values in the high gradient 

areas. Confinement of the flowing channel by freezing-in from the sides as shown in figures 4 and 

5 reduces the conveyance area, resulting in higher mid-channel velocities than would otherwise be 

encountered. The observed and simulated mid-channel velocities routinely exceed 0.6 m/s in high 

gradient areas. Figure 8 shows the observed and simulated velocity profile at the discharge 

measurement transect employed on January 24, 2006. See appendix 2 for measurement details. 

13
Note that the higher edge velocities are partly an artifact of the model’s current lack of a 

mechanism to freeze the ice to be bed. These “shoulder” velocities represent areas at the edge 

where the model floated the ice and allowed a portion of the discharge to pass through areas that 

were observed frozen to the bed. The smoothed nature of the simulated velocity profile and the 

higher velocities near the edges are also partly an artifact of the spatial averaging inherent in twodimensional models. 

DISCHARGE TRANSECT VELOCITY PROFILES

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

8 9 10 11 12 13 14 15

STATION, IN METERS

VELOCITY (m/s) 

OBSERVED

SIMULATED

Figure 8. Observed and simulated velocity profile obtained for discharge of 0.608 m3/s observed 

January 24, 2006. 

Figures 9 and 10 show the spatial depth and velocity distribution simulated for the 0.608 

m3

/s discharge (21.48 ft3/s) encountered January 24, 2006. One of the backwater areas isolated by 

the constructed low dikes (indicated in fig. 9) has sufficient depth that approximately 0.4 m of 

water remain unfrozen under the 0.6 m ice cover. 

14
Figure 9. Depth distribution under ice cover for discharge of 0.608 m3

/s, January 24, 2006. 

15
Figure 10. Velocity distribution under ice cover for discharge of 0.608 m3

/s, January 24, 2006; Qin = 

inflow discharge, Qout = simulated outflow discharge. 

River2D Performance With Ice Cover 

Addition of ice cover increases the computational burden for the River2D model. In 

particular, edges of the wetted area appear to have two varieties of computational difficulties that 

must be accommodated when applying the model. First, convoluted edges and large time steps are 

difficult for the model to handle as presently formulated. The north bank of the river bend near the 

middle of the site is riprapped to protect the adjacent highway. The original computational mesh 

follows the riprap contours in great detail. This detail produced locally infeasible results when ice 

cover was added. To remedy this problem, the riprap contours were smoothed and a new mesh was 

developed for that area. Second, it was also necessary to limit the time step attempted in the model 

to prevent numerical infeasibilities, so the maximum time step was constrained by switching from 

the direct solver to the iterative Generalized Minimal Residual solver. (Steffler and Blackburn, 

2002 p 40-41) With these changes the model ran without encountering infeasible conditions. 

As currently formulated, the River2D model does not have the ability to attach the ice cover 

to the river bed. As a result, the mode may predict that ice floats in some edge areas where the ice 

is actually frozen to the bed. This phenomena results in a thin film of water (often less than 0.03 m 

in depth) occurring in some edge areas and in calculation of unrealistically high velocities for those 

areas. The areas subject to this phenomenon are unstable, such that the computational elements 

16
with such shallow depths and unrealistic velocities change with each iteration of the solution 

procedure. Because of the very small depths involved, the mass flux of these areas was deemed to 

be insignificant to the overall solution. Further, areas of such depths are rated as zero habitat value 

and excluded from the habitat sum due to being too shallow to satisfy the suitability criteria. 

Therefore, the unusual edge behavior did not influence the predicted amount of habitat. 

Effects of Ice Cover on Flow Characteristics 

To evaluate the overall effects of ice cover, the River2d model was run at 0.566 m3/s (20 

ft3

/s) for both open water and ice covered conditions. Recall that depth in the presence of ice cover 

is defined as the distance from the bed to the bottom of the ice. Figure 11 shows reductions in the 

total area occupied by all but one of eleven depth bins due to the presence of ice. Figure 12 shows 

that the area occupied by the lowest velocity bin decreases substantially in the presence of ice and 

increases in areas occupied by velocities greater than 0.5 m/s. These results are consistent with the 

observed ice formation from the sides toward the middle of the channel noted earlier. That is, as the 

ice became thicker and more of the channel edge became frozen to the bed, more low-velocity 

areas were occupied by ice, forcing the flow into the remaining conveyance area at higher 

velocities. In summary, ice produced less flow area and higher velocities. 

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

0< D 

≤0.15

0.15< D 

≤0.30

0.30< D 

≤0.46

0.46< D 

≤0.61

0.61< D 

≤0.76

0.76< D 

≤0.91

0.91< D 

≤1.07

1.07< D 

≤1.22

1.22< D 

≤1.37

1.37< D 

≤1.52

D >1.52

DEPTH BIN (m)

AREA (m2

)

ICE

NO ICE

Figure 11. Area occupied by depth increments with and without ice at 0.566 m3/s; D = depth. 

17
0

500

1,000

1,500

2,000

2,500

3,000

3,500

V 

≤0.15

0.15< V 

≤0.30

0.30< V 

≤0.46

0.46< V 

≤0.61

0.61< V 

≤0.76

0.76< V 

≤0.91

0.91< V 

≤1.07

1.05< V 

≤1.22

1.22< V 

≤1.37

V >1.37

VELOCITY BIN (m/s)

AREA (m2

)

ICE

NO ICE

Figure 12. Area occupied by velocity increments with and without ice at 0.566 m3/s; V = velocity. 

Physical Effects of Winter Release 

A release of 0.283 m3/s (10 ft3/s) from reservoirs upstream of the Kinikinik study site was 

maintained during winter 2005–2006. To evaluate the effects on the stream of this release, the 

calibrated River2D model was run for the observed January 24, 2006, condition (0.608 m3

/s) and an 

assumed condition of 0.325 m3

/s; the observed condition less 0.283 m3/s. 

Reducing the discharge at the study site from 0.608 m3

/s to 0.325 m3/s when subject to the 

same ice cover reduces the wetted area from 4,092 m2

 to 3,559 m2. Figures 13 and 14 show the 

changes in depth and velocity bin area. Note that reducing the discharge reduces the area occupied 

by all depth bins and the area occupied by all but the lowest velocity bin. Thus, in general, reducing 

the discharge by 0.283 m3

/s produces across the board reductions in the depths and velocities. 

18
0

200

400

600

800

1,000

1,200

1,400

0< D <0.15

0.15< D <0.30.3< D <0.46

0.46< D <0.610.61< D <0.760.76< D <0.910.91< D <1.071.07< D <1.221.22< D <1.371.37< D <1.52

D >1.52

DEPTH BIN (m)

AREA (m2

)

0.608 m3/s 

0.325 m3/s 

Figure 13. Area occupied by depth increments at 0.608 m3/s and 0.325 m3/s. 

19
0

200

400

600

800

1000

1200

1400

1600

1800

V < 0.15

0.15< V <0.300.30< V <0.460.46< V <0.610.61< V <0.760.76< V <0.910.91< V <1.071.05< V <1.221.22< V <1.37

V >1.37

VELOCITY BIN (m/s)

AREA (m2

)

0.608 m3/s 

0.325 m3/s 

Figure 14. Area occupied by velocity increments at 0.608 m3/s and 0.325 m3/s. 

Winter Habitat 

Freshwater salmonids exhibit a wide range of behaviors in response to winter conditions. 

Some have been observed maintaining active feeding and migration behavior in winter. (Cunjak 

and Power, 1986; Cunjak, 1996; Jakober, and others 1998; Simpkins and others 2000; Lindstrom 

and Hubert, 2004) However, winter conditions are also known to produce high fish mortality. 

(Annear, and others 2002) Other authors observed fish entering pools created by beaver dams and 

preferring those pools throughout winter. (Chisholm, and others, 1985; Cunjak, 1996; Jakober, and 

others, 1998,) Harper and Farag (2004) observed that cutthroat trout used pools during periods of 

cold temperatures and runs when water temperatures exceeded 1° C. Cunjak (1996, p. 273) points 

out “winter habitat selection often differs from that in summer, generally involving movement to 

areas with lower water velocity (day and night).” Due to the wide range of reported behaviors, it is 

difficult to predict the specific behavioral responses to temperature and winter ice cover of brown 

and rainbow trout in the Cache la Poudre River. Cunjak (1996) provides some guidance when he 

states that areas of lower velocity will be selected to minimize energy expenditure when exposed to 

lower winter temperatures. 

Few habitat suitability criteria (HSC) have been developed specifically for winter 

conditions, particularly under ice cover, due to the rigor of obtaining significant samples. Cunjak 

20
and Power (1986, p. 1975) observe: “In temperate latitude streams subject to freezing conditions 

for considerable periods (months) over the winter, there exists no published record of salmonid 

behaviour based on underwater observations.” In the same article, Cunjak and Power describe 

winter fish observations, but they worked in a stream that did not freeze over. Thus, a quantitative 

evaluation of habitat with ice cover in the Poudre River requires analysis of the physical conditions 

present in the stream and inference from relative differences in calculated habitat derived from 

summer or at least open water HSC. 

Thomas and Bovee (1993) developed habitat suitability for adult brown trout (Salmo trutta) 

and rainbow trout (Oncorhyncus mykiss) for the South Platte River and found the criteria to be 

transferable to the Cache la Poudre River. Applying those criteria to the Kinikinik study site, and 

assuming that all substrate in the study site are fully suitable for over-wintering adult trout, yields 

the following habitat results (table 2): 

Table 2. Habitat area (Weighted Usable Area) for two discharges at the Kinikinik site for ice 

conditions equivalent to those observed January 24, 2006 and for open channel conditions.

Ice condition 

Simulated discharge 

m3

/s Brown trout adults m2

Rainbow trout adults 

m2

Ice 0.608 660.66 532.25 

Ice 0.325 422.87 346.18 

No ice 0.608 831.93 728.46 

No ice 0.325 565.67 507.41 

When using Weighted Usable Area based on the Thomas and Bovee (1993) HSC as the 

habitat metric, reducing flow without ice cover by approximately one-half of the discharge 

observed on January 24, 2006 produces a reduction of about 32 percent in the calculated habitat 

metric. When flow under ice cover is reduced by the same amount, habitat is reduced by about 35 

percent. As noted earlier, this example applies summer criteria to winter conditions. It serves as an 

illustration of the relative habitat changes occurring when winter flows are reduced. That is, overall 

an approximate 47 percent reduction in flow produces an approximate 33 percent reduction in 

habitat area, with or without ice. Definitive description of winter HSC and therefore habitat use 

under ice cover would require development of criteria specifically designed for that purpose. 

Cunjak (1996) and Lindstrom and Hubert (2004) note that winter habitat selection involves 

movement into areas with lower velocity and(or) greater depth. These observations (though 

qualitative in nature) suggest that a patch-type habitat metric may apply to the Kinikinik site in 

winter. The calculated velocity bins shown in figure 14 suggest that reducing the discharge may 

increase the area of the stream that contains velocities less than 0.15 m/s. However, as delineated, 

the velocity bins do not differentiate between the narrow band that can occur at the edge, between 

the bottom of the ice and the bed, and the areas that would be classified as pools.2

 Therefore, pool 

habitat was evaluated using the pool habitat criteria defined in table 1. 

Using these pool patch criteria results in total pool areas shown in table 3, when evaluated 

for the observed January 24, 2006, condition (0.608 m3

/s) and a reduced flow of 0.325 m3/s. Note 

that for a maximum velocity threshold of 0.15 m/s the total pool area increases with decreasing 

discharge when the depth limit is relaxed to 0.5 or 0.3 meters (rows 4 and 5), as illustrated in figure 

 

2

 In this application, “pool” refers to an area of adequate depth and lower velocities that provides resting shelter during 

extreme low temperature conditions. Pool patches are defined as areas with v<V and d>D, where V and D are defined 

pool velocity and depth thresholds and v and d are calculated velocity and depth for a patch. 

21
15. This increase is due to an increase in mid-channel velocity at the higher discharge exceeding 

the 0.15 m/s threshold without sufficient increase in slow area to offset the loss of mid-channel 

slow habitat. For all other cases, a decrease in discharge of 0.283 m3

/s from the observed condition 

results in a decrease in pool area, as defined by the depth and velocity thresholds. 

Table 3. Area of low velocity and increased depth based on different pool criteria, results derived 

from criteria provided by U.S. Forest Service are shown in bold. 

Case 

Minimum 

depth 

(m) 

Maximum 

velocity 

(m/s) 

Pool area at 

0.608 m3

/s 

(m2

) 

Pool area at 

0.325 m3

/s 

(m2

) 

1 1.5 0.15 4.51 0.0 

2 1.0 0.15 81.63 59.43 

3 0.75 0.15 209.01 197.72 

4 0.5 0.15 463.04 499.23 

5 0.3 0.15 756.47 983.26 

6 1.5 0.3 4.51 0.0 

7 1.0 0.3 83.9 59.43 

8 0.75 0.3 277.65 202.28 

9 0.5 0.3 753.47 591.98 

10 0.3 0.3 1,464.8 1,341.5 

0

200

400

600

800

1000

1200

1.5 1 0.75 0.5 0.3

MINIMUM DEPTH (m)

AREA (m2

)

0.608 m3/s 

0.325 m3/s 

Figure 15. Habitat area under ice for V<0.15 m/s. 

22
The change in pool habitat occurs in the two deep pools in the study area (see figure 16). 

The reduction in pool area occurring from 0.608 m3

/s to 0.325 m3/s for all but the two shallow pool 

definitions (cases 4 and 5, table 3; fig. 15) illustrate an overall tendency for sheltered pool areas to 

change in concert with discharge regardless of the exact pool criteria. Based on the Cunjak (1996) 

and Lindstrom and Hubert (2004) results, it would appear that the two deep pools provide the most 

reliable winter habitat in the Kinikinik study site. Under-ice observations of fish in this study area, 

the exact kind of data that is missing, would be required to confirm the actual degree of habitat 

usage. 

DEPTH > 1 m

VELOCITY < 0.3 m/s

Figure 16. Pool areas at 0.608 m3

/s using the Forest Service criteria; gray lines are topographic 

contours. 

Discussion 

Chisholm, and others, (1985) did not observe anchor or frazil ice in a study in the Snowy 

Range, Wyo. (approximately 100 km north of the Cache la Poudre River study site), in areas where 

the stream was covered with ice or snow. Neither frazil nor anchor ice was observed in the open 

leads measured during this study. The form of the habitat model used in this study inherently 

assumes the water column is free of obstructions not included in the topographic data. In the 

23
present context, it inherently assumes frazil ice is not present. The hydrodynamic model 

formulation does deal with ice cover over the stream and thus appears to be consistent with the 

conditions observed in the field. 

At the current state of development, the River2D model requires the ice cover to be defined 

as input to the hydrodynamic modeling process. In this situation, the ice cover developed from 

January 24, 2006, observations (0.608 m3

/s discharge) was assumed to be representative of the ice 

cover present at 0.325 m3

/s. The extent and thickness of the actual ice cover without a winter 

release from upstream reservoirs would, of course, be different. The degree of difference can only 

be determined empirically. For the purposes of this study using the ice cover generated from 

January 24, 2006 observations was judged to be adequate to illustrate the overall magnitude and 

location of depth and velocity conditions at the lower discharge. 

The 0.283 m3

/s release from headwater reservoirs is currently maintained throughout the 

winter. The ice cover observed on January 24, 2006, appears to be the result of cold periods that 

occurred in December and January. Similar cold temperatures continued with a few interruptions 

until the last third of March 2006. Because the current management practice is based on a fixed 

release from upstream reservoirs, and because the cold conditions continued from mid-December 

through mid-March, the assumption that the Jananuary 24, 2006 condition is representative of 

overall winter hydrodynamics seems justified. 

This analysis of fish habitat under winter conditions does not consider stream discharge 

needed to maintain connectivity between pools. Observation of ice formation during winter 2005–

2006 suggests that the flowing stream will maintain adequate space between the ice and the bed to 

convey the streamflow. Abrupt increases in discharge are likely to result in flow over the surface of 

the ice. When drilling through the ice we encountered areas of over-ice where 3–5 cm of ice had 

formed on top of the consolidated ice cover resulting in distinct layers. It appears likely that a larger 

sustained discharge would result in freezing and erosion of the bottom of the ice until equilibrium 

between the discharge and ice thickness was reached. Thus flow of water between the pools is 

likely to be maintained; however, thorough evaluation of the capacity of fish to move through the 

high velocities and low temperatures encountered under the ice would require more information. 

In addition to low water velocities requiring less energy expenditure, a portion of the 

definition of a “pool” common in fisheries science is based on having sufficient depth that fish are 

less subject to avian predation. With complete ice cover, that portion of the definition may be 

removed for this study site. Redefining “pool” as “pool under ice” may allow depths as shallow as 

0.3 m with slow velocities to be classified as pools. Applying such a definition will increase the 

amount of pool habitat calculated for both discharges. For some “pool” criteria definitions, 

reduction of velocities in shallow pools may result in increased area with a decrease in discharge. 

However, overall there is an apparent increase in pool habitat obtained by increasing the discharge. 

Due to the process of freezing-in from the stream margins, shallow edge habitat is reduced 

in winter regardless of flow. Therefore, because of the nature of freezing dynamics, neither 

increasing nor decreasing the flow will enhance edge habitats in a stream with stable winter flow, 

such as the stream observed in this study,. 

Using a Weighted Usable Area metric leads to the conclusion that, within the range of 

conditions simulated, increased winter flow increases the habitat area for both brown and rainbow 

trout. With the exception of combinations of shallow depth and low velocity threshold, the same 

conclusion is borne out for pool habitats. An extensive study of brown and rainbow trout subjected 

to ice cover such as was observed at the Kinikinik site would be necessary to objectively determine 

the actual occupancy of different habitats by over-wintering trout in the Cache la Poudre River. 

A survey of channel types for the substantial portion of the Cache la Poudre River 

influenced by the winter release was not conducted as part of this study. Thus, the precise length of 

24
the river represented by the Kinikinik study site is unknown. The Kinikinik site is in a portion of 

the Poudre River Valley that was glaciated and is representative of the channel characteristics 

observed upstream of the terminal moraine known as Home Moraine. A study of ice formation and 

break-up in steeper gradient sections of the river would provide a more representative description 

of those sections. However, the general occlusion of the stream margins by ice formation would 

occur in all portions of the river subject to extended periods of sub-zero temperatures such as those 

that occurred in December 2005, and January and February 2006. 

Products of this Study 

River2D files containing the solutions for 0.608 and 0.325 m3/s; the ice cover generated for 

January 24, 2006; the habitat suitability criteria based on the Thomas and Bovee (1993) study; and 

the pool criteria used herein are products of this study. These files and brief instructions about 

loading them to River2D are provided to the U.S. Forest Service with this report. The Forest 

Service may then use these files to reevaluate weighted usable area for the flow-under-ice 

conditions described herein with alternative habitat suitability criteria. 

References Cited 

Annear, T.C., Hubert, W., Simpkins, D., and Hebdon L., 2002, Behavioral and physiological 

response of trout to winter habitat in tailwaters in Wyoming, USA, Hydrological Processes, v. 

16, no. 4, p. 915–924. 

Bovee, K.D., 1982, A guide to stream habitat analysis using the instream flow incremental 

methodology: Instream Flow Information Paper No. 12, U.S. Fish and Wildlife Service 

WS/OBS–82/26. 

Bovee, K.D., Lamb, B.L., Bartholow, J.M., Stalnaker, C.B., Taylor, J., and Henriksen, J., 1998, 

Stream habitat analysis using the instream flow incremental methodology: Fort Collins, Colo., 

U.S. Geological Survey. Information and Technology Report USGS/BRD/ITR–1998-004. 

Bowen, Z., Bovee, K., and Waddle, T., 2003, Effects of flow regulation on shallow-water habitat 

dynamics and floodplain connectivity: Transactions of the North American Fisheries Society, v. 

132, p. 809–823. 

Chisholm, I.A., Hubert, W.A., and Wesche, T.A., 1985, Winter stream conditions and brook trout 

habitat use in the Snowy Range, Wyoming: Laramie, Wyo, University of Wyoming, Wyoming 

Water Research Center, WWRC-85-15, 113 pp. 

Cunjak, R.A., 1996, Winter habitat of selected stream fishes and potential impacts from land-use 

activity: Canadian Journal of Fisheries and Aquatic Sciences, v. 53, supplement 1, p. 267–282. 

Cunkak. R.A. and Power, G., 1986, Winter habitat utilization by stream resident brook trout 

(Salvelinus fontinalis) and brown trout (salmo trutta): Canadian Journal of Fisheries and Aquatic 

Sciences, v. 43, p.1970–1981. 

25
Harper, D.D. and Farag, A.M., 2004, Winter habitat use by cutthroat trout in the Snake River near 

Jackson, Wyoming. Transactions of the American Fisheries Society, v. 133, p.15–25. 

Jakober, M.J., McMahon, T.E., Thurow, R.F., and Clancy, C.G., 1998, Role of stream ice on fall 

and winter movements and habitat use by bull trout and cutthroat trout in Montana headwater 

streams: Transactions of the American Fisheries Society, v. 127, p. 223–235. 

Lindstrom, J.W., and Hubert, W.A., 2004, Ice processes affect habitat use and movements of adult 

cutthroat trout and brook trout in Wyoming foothills stream: North American Journal of Fisheries 

Management, v. 24, p.1341–1352. 

Milhous, R., Updike, M., and Schneider, D., 1989, Physical habitat simulation system reference 

manual -- version II: Washington, D.C., U.S. Fish and Wildlife Service, Biological Report 

89(16), 403 p. 

Simpkins, D.A., Hubert, W.A., and Wesche, T., 2000, Effects of fall to winter changes in habitat 

and frazil ice on the movements and habitat use by juvenile rainbow trout in a Wyoming 

tailwater. Transactions of the American Fisheries Society, v. 129, p.101–118. 

Steffler, P. and Blackburn, J., 2002, River2D, Two–dimensional depth averaged model of river 

hydrodynamics and fish habitat. Introduction to depth averaged modeling and user's manual: 

Edmonton, Alberta, Canada, University of Alberta, 119 p. 

http://www.river2d.ualberta.ca/software/River2D.pdf. 

Thomas, J.A., and Bovee, K.D., 1993, Application and testing of a procedure to evaluate 

transferability of habitat suitability criteria—Regulated Rivers: Research and Management, v. 8, 

p. 285–294. 

26
Appendix 1. Habitat Suitability Criteria from Thomas and Bovee (1993) 

Table 1-1. Brown Trout adults active. (Data from South Platte River, Colorado.) 

[Continuous criteria from exponential polynomial curves, English units translated to metric, 1 = no cover, 2 = visual 

isolation, 3 = velocity shelter, 4 = combination: velocity shelter + visual isolation.] 

Mean 

velocity 

(m/s) SI 

Depth 

(m) SI Cover SI 

0.000 0.33 0.000 0.00 1 0.72 

0.250 0.81 0.201 0.04 2 0.72 

0.326 0.92 0.290 0.10 3 1.00 

0.402 1.00 0.357 0.19 4 1.00 

0.497 1.00 0.686 0.80 

0.591 0.92 0.817 0.94 

0.914 0.44 0.905 1.00 

1.064 0.26 1.036 1.00 

1.140 0.19 1.149 0.92 

1.274 0.11 1.411 0.58 

1.426 0.06 1.588 0.34 

1.692 0.02 1.698 0.22 

1.768 0.00 1.829 0.12 

100.000 0.00 2.103 0.00 

100.000 0.00 

27
Appendix 1. Concluded 

Table 1-2. Rainbow Trout adults active. (Data from South Platte River, Colorado.) 

[Continuous criteria from exponential polynomial curves, English units translated to metric, 1 = no cover, 2 = visual 

isolation, 3 = velocity shelter, 4 = combination: velocity shelter + visual isolation.] 

Mean 

velocity 

(m/s) SI 

Depth 

(m) SI Cover SI 

000 0.31 0.000 0.00 1 1 

0.171 0.55 0.238 0.07 2 1 

0.357 0.86 0.393 0.19 3 1 

0.433 0.95 0.588 0.44 4 1 

0.488 1.00 0.820 0.81 

0.579 1.00 1.015 1.00 

0.655 0.94 1.134 1.00 

0.747 0.81 1.250 0.93 

0.972 0.37 1.561 0.59 

1.064 0.22 1.756 0.38 

1.195 0.09 1.951 0.23 

1.308 0.03 2.222 0.11 

1.494 0.00 2.536 0.05 

100.000 0.00 2.847 0.03 

3.048 0.00 

100.000 0.00 

28
Appendix 2. Discharge and Ice Thickness Measurements November 2005–

February 2006, Transcribed Field Notes 

Table 2-1. Discharge measurement November 22, 2005. 

[Discharge measurement notes transcribed from field book, note: English units translated to metric, RB = right bank, 

LEW = left edge of water.] 

Stage influenced by ice 

Ice broken to facilitate discharge measurement 

Bridge Pier NW to bank perpendicular to flow 

Marsh # s/n 2003632 

Pygmy # 

Start time: 11:40 a.m. 

End time: 12:22 p.m. 

Pygmy Marsh 

Station (m) Depth (m) 

velocity 

(m/s) 

velocity 

(m/s) Note 

0.3 0.305 0.00 0.00 Pier edge RB 

0.7 0.564 0.01 0.01 

0.9 1.189 0.08 0.08 

1.3 1.128 0.155 0.19 

2.0 0.914 0.29 0.25 

2.5 0.823 0.40 0.4 

3.0 0.732 0.37 0.32 

3.5 0.610 0.53 0.54 

4.0 0.561 0.44 0.43 

4.5 0.518 0.37 0.41 

5.0 0.472 0.34 0.3 

5.5 0.411 0.24 0.27 

6.0 0.366 0.26 0.25 

6.5 0.335 0.19 0.19 

7.0 0.305 0.07 0.07 

7.5 0.244 0.03 0.05 

8.0 0.091 0.02 0.01 

9.5 0.000 0.00 0.00 LEW 

 

 

29
Table 2-2. Discharge measurement December 12, 2005. 

[Discharge measurement notes transcribed from field book, note: English units translated to metric, frzn = frozen, 

WSL = water surface level, xsec = transect, UTM = universal transverse Mercator.] 

Start Time 10:18 a.m. 

End Time 10:50 a.m. 

Marsh # s/n 2003632 

Q xsec located in partially open water from approximate UTM locations 

 437666 4507048 

to 437664.5 4507041.5 

width: 6.67 meters 

Area adjusted for ice, measured through chopped holes where covered 

 Station 0 solid ice N. side 

Station 

(m) Depth (m) 

Velocity 

(m/s) 

Distance 

from 

stream 

bed to 

bottom of 

ice (m) 

Velocity 

depth 

Calculated 

ice 

thickness 

(m) Notes 

0.00 - - 0.25 frzn to bed 

1.20 0.00 - 0.33 frzn to bed 

1.70 0.39 0.00 0.28 0.5d 0.11 

2.35 0.40 0.57 0.28 0.5d 0.12 

3.00 0.49 1.05 0.6d open 

3.50 0.45 0.95 0.6d open, WSL = 

top of ice 

4.00 0.50 0.65 0.6d open 

4.50 0.43 0.60 0.29 0.5d 0.14 

5.00 0.40 0.28 0.25 0.5d 0.15 

5.50 0.37 0.05 0.03 0.5d 0.34 

6.50 0.25 - 0.25 frzn to bed 

30
Table 2-3. Discharge measurement January 24, 2005. 

[Discharge measurement notes transcribed from field book, note: English units translated to metric, velocity depth as 

fraction of active depth (to bottom of ice), frzn = frozen, L. Pin @ Root = left bank zero location at a root near the top 

of the bank.] 

Start Time: 11:40 a.m. 

End Time: 12:25 p.m. 

Marsh McBirney # 2003632 

Note; water depth under ice is the distance between bottom of ice and bed 

Depth of ice is measured to top of water 

(1) L. Edge of channel before ice was removed 

(2) R. Edge of channel before ice was removed 

Clarification: these references are to edges of open lead 

Station 

(m) 

Depth of 

flow(m) 

Velocity 

(m/s) 

Measured 

ice 

thickness 

(m) Velocity depth Notes 

0.000 L.Pin @ root 

0.305 edge of ice 

4.572 edge of bar 

5.913 edge of bar/ice 

9.266 frzn to bed 

9.754 0.091 0.000 0.366 0.5d under ice 

10.211 0.213 0.366 0.274 0.2d 

0.8d 

" 

10.668 0.244 0.480 0.305 0.2d 

0.8d 

" 

11.125 0.457 0.792 0.6d open water (1) 

11.582 0.518 0.564 0.6d open 

12.040 0.488 0.579 0.6d open water (2) 

12.497 0.457 0.433 0.183 0.6d under ice 

13.106 0.152 0.000 0.366 0.2d 

0.8d 

" 

13.564 0.061 0.000 0.518 0.5d " 

14.173 0.000 0.610 0.5d frzn to bed 

16.764 edge of ice 

31
Table 2-4. Discharge measurement February 23, 2005. 

[Discharge measurement notes transcribed from field book, note: English units translated to metric, Velocity depth as 

fraction of active depth (to bottom of ice), L. Edge = left edge, R. edge = right edge, Est = estimated, over ice = ice that 

has formed after water flowed over the established ice surface.] 

Start Time: 12:15 p.m. 

End Time: 12:48 p.m. 

Marsh McBirney #2003632 

stationing faces upstream 

 

Station (m) Depth (m) 

Velocity 

(m/s) 

Measured 

ice 

thickness 

 (m) Velocity depth Notes 

0 L. edge ice at 

bank 

0.5 0 0.34 ice to bed 

0.75 0.400 0 0.31 edge of water 

1.05 0.427 0.56 0.15 0.6d 4 cm over ice 

1.55 0.518 0.86 0.08 0.6d 2 cm over ice 

2.05 0.518 0.88 0.22 0.6d 2 cm over ice 

2.55 0.488 0.76 0.11 0.6d 2 cm over ice - 

edge open water 

3.05 0.488 0.74 0.6d open water 

3.9 0.579 0.5 0.31 0.6d 3 cm over ice - 

edge open water 

4.35 0.518 0.02 0.44 0.6d 2 cm over ice 

4.85 0.430 0 0.33 3 cm over ice 

5.5 est ice to bed 

9.3 R. edge of ice at 

bar 

32
Appendix 3. Ice Thickness Measurements January 24, 2006 

Table 3-1. Cross Section 1 from survey grade Global Positioning System (GPS). 

[GPS: UTM Zone 13 N, Datum: WGS 1984, UTM = universal transverse Mercator, WGS 1984 = world geodetic 

system 1984 standard ellipsoid, Stationing adjusted to match tagline measurements, point names and controller notes as 

logged in field, xsl = transect line, lie = left edge of ice, rie = right edge of ice, mid chl = mid–channel, wsl = water 

surface elevation.] 

Point X Y Z (m) 

Controller 

notes 

Station 

(m) 

012406p1 437663.577 4507049.964 2356.221 xs1 lie 7.500 

012406p13 437663.544 4507049.858 2356.172 xs1 lie 23 7.611 

012406p2 437662.989 4507047.905 2356.138 xs1 top ice 9.641 

012406p6 437662.641 4507046.424 2356.123 xs1 top ice 11.162 

012406p9 437662.147 4507044.194 2356.119 xs1 top ice 13.445 

012406p11 437661.956 4507042.659 2356.149 xs1 ice to bed 14.983 

012406p12 437661.672 4507040.353 2356.197 xs1 rie 17.298 

012406p5 437662.845 4507046.620 2355.926 xs1 bot ice 10.923 

012406p8 437662.167 4507044.095 2355.848 xs1 bot ice 13.536 

012406p4 437663.158 4507047.745 2355.631 xs1 bed 9.758 

012406p7 437662.465 4507045.363 2355.507 xs1 mid chl 12.233 

012406p10 437662.056 4507042.647 2355.673 xs1 bed 14.973 

012406p3 437663.009 4507047.903 2356.060 xs1 wsl 9.638 

012406p14 437662.561 4507046.152 2356.027 xs1 wsl 11.445 

 

33
Table 3-2. Cross Section 1 from tag line. 

[Top of Ice elevations obtained by calculating best fit to GPS data, bottom of ice by subtracting thickness, L. Pin @ 

Root = left bank zero location at a root near the top of the bank, frzn = frozen, edge of ice = location where earth, sand 

or gravel bank intersected with ice cover.] 

Station 

(m) 

Depth 

(m) 

Calculated 

elevation 

(m) 

Ice 

thickness 

(m) 

Top of ice 

(m) 

Point 

notes 

Bottom of 

ice (m) WSL (m) 

0.000 L.Pin @ root 

0.305 2356.120 0.000 edge of ice 

4.572 2356.220 edge of bar 

5.913 2356.120 0.000 2356.220 edge of bar/ice 2356.120 

9.266 2355.721 0.399 2356.142 frzn to bed 2355.721 

9.754 0.091 2355.663 0.366 2356.137 under ice 2355.754 

10.211 0.213 2355.632 0.274 2356.131 " 2355.846 

10.668 0.244 2355.571 0.305 2356.125 " 2355.815 

11.125 0.457 2355.663 2356.120 open 2356.120 2356.059 

11.582 0.518 2355.602 2356.120 open 2356.059 

12.040 0.488 2355.632 2356.120 open 2356.120 2356.059 

12.497 0.457 2355.480 0.183 2356.120 under ice 2355.937 

13.106 0.152 2355.602 0.366 2356.120 " 2355.754 

13.564 0.061 2355.541 0.518 2356.120 " 2355.602 

14.173 0.000 2355.510 0.610 2356.128 frzn to bed 2355.510 

16.764 2356.120 0.000 2356.197 edge of ice 2356.120 

34
Table 3-3. Cross Section 2 from survey grade Global Positioning System (GPS). 

[GPS: UTM Zone 13 N, Datum: WGS 1984, UTM = universal transverse Mercator, WGS 1984 = world geodetic 

system 1984 standard ellipsoid Stationing adjusted to match tagline measurements, point names and controller notes as 

logged in field, xs 2 = transect 2 , i = ice thickness in cm, d = depth from ice surface to bottom in cm, ice bottom and 

bed elevations obtained by subtracting thickness and depth from ice top elevation.] 

Point X Y Ice top (m) 

Controller 

notes 

Station 

(m) 

Ice bottom 

by 

subtraction 

(m) 

Bed by 

subtraction 

(m) 

012406p26 437598.364 4507088.309 2356.581 xs 2 i24 d 0 1.562 2356.341 2356.341 

012406p25 437598.407 4507087.536 2356.581 xs 2 i47 d 93 2.334 2356.111 2355.651 

012406p24 437598.427 4507087.540 2356.581 xs 2 i47 d 93 2.336 2356.111 2355.651 

012406p23 437598.382 4507087.156 2356.574 xs 2 i28 d 93 2.711 2356.294 2355.644 

012406p22 437598.362 4507086.554 2356.596 xs 2 i20 d101 3.305 2356.396 2355.586 

012406p21 437598.730 4507085.150 2356.581 xs 2 i20 d 98 4.745 2356.381 2355.601 

012406p20 437598.657 4507085.212 2356.577 xs 2 i20 d 98 4.816 2356.377 2355.597 

012406p19 437598.722 4507085.142 2356.577 xs 2 i38 d 88 4.893 2356.197 2355.697 

012406p18 437598.748 4507084.800 2356.576 xs 2 i36 d 83 5.236 2356.216 2355.746 

012406p17 437598.776 4507084.381 2356.583 xs 2 i50 d 77 5.655 2356.083 2355.813 

012406p16 437598.848 4507083.906 2356.590 xs 2 i50 d64 6.135 2356.090 2355.950 

35
Table 3-4. Cross Section 3 from survey grade Global Positioning System (GPS). 

[GPS: UTM Zone 13 N, Datum: WGS 1984, UTM = universal transverse Mercator, WGS 1984 = world geodetic 

system 1984 standard ellipsoid Stationing adjusted to match tagline measurements, point names and controller notes as 

logged in field, xs 2 = transect 2 , i = ice thickness in cm, d = depth from ice surface to bottom in cm, ice bottom and 

bed elevations obtained by subtracting thickness and depth from ice top elevation.] 

Point X Y Ice top (m) 

Controller 

notes 

Station 

(m) 

Ice bot by 

subtraction 

(m) 

Bed by 

subtraction 

(m) 

012406p38 437532.084 4507073.649 2356.954 xs3i54d 7.6f 6.515 2356.414 2354.638 

012406p37 437533.741 4507071.367 2356.910 xs3i66d 7.2f 9.334 2356.250 2354.715 

012406p36 437535.665 4507068.832 2356.913 xs3i60d 5.5f 12.517 2356.313 2355.237 

012406p35 437537.932 4507066.173 2356.902 xs3i46d 5.5f 16.007 2356.442 2355.226 

012406p34 437539.362 4507064.292 2356.934 xs3i51d 3.8f 18.369 2356.424 2355.776 

36
37

Table 3-5. Cross Section 4 from survey grade Global Positioning System (GPS). 

[GPS: UTM Zone 13 N, Datum: WGS 1984, UTM = universal transverse Mercator, WGS 1984 = world geodetic 

system 1984 standard ellipsoid Stationing adjusted to match tagline measurements, point names and controller notes as 

logged in field, xs 2 = transect 2 , i = ice thickness in cm, d = depth from ice surface to bottom in cm, ice bottom and 

bed elevations obtained by subtracting thickness and depth from ice top elevation.] 

Point X Y Ice top (m) 

Controller 

notes 

Station 

(m) 

Ice bot by 

subtraction 

(m) 

Bed by 

subtraction 

(m) 

012406p27 437525.741 4507054.530 2357.067 xs 4 lie 0.000 2356.950 2356.950 

012406p28 437528.352 4507055.054 2357.018 xs 4 icetobed 2.614 2356.830 2356.830 

012406p29 437529.292 4507055.324 2356.934 x4icetobed37 3.556 2356.564 2356.564 

012406p30 437531.950 4507055.204 2357.002 xs4 i44 d59 6.213 2356.562 2356.412 

012406p31 437533.116 4507055.241 2356.974 xs4 i37 d89 7.379 2356.604 2356.084 

012406p32 437533.761 4507055.273 2356.952 xs4 i38 d88 8.024 2356.572 2356.072 

012406p33 437534.707 4507055.183 2357.013 xs4 i2 d92 8.970 2356.813 2356.093