Prepared in cooperation with the Northern Shenandoah Valley Regional Commission, Central

Shenandoah Valley Planning District Commission, and Virginia Commonwealth University

South Fork Shenandoah River Habitat-Flow Modeling to 

Determine Ecological and Recreational Characteristics 

during Low-Flow Periods

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5081
Cover: Bedrock Run between Newport and Alma, South Fork Shenandoah River, Virginia

All photographs in this report are courtesy of the author, Jennifer Krstolic, U.S. Geological Survey, 

Virginia Water Science Center
South Fork Shenandoah River Habitat-Flow 

Modeling to Determine Ecological and 

Recreational Characteristics during 

Low-Flow Periods

By Jennifer L. Krstolic and R. Clay Ramey

Prepared in cooperation with the 

Northern Shenandoah Valley Regional Commission, 

Central Shenandoah Valley Planning District Commission, and 

Virginia Commonwealth University

Scientific Investigations Report 2012–5081

U.S. Department of the Interior

U.S. Geological Survey
ISBN 978–1–4113–3419–9

U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, 

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U.S. Government.

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

reproduce any copyrighted materials contained within this report.

Suggested citation:

Krstolic, J.L., and Ramey, R.C., 2012, South Fork Shenandoah River habitat-flow modeling to determine ecological 

and recreational characteristics during low-flow periods: U.S. Geological Survey Scientific Investigations Report 

2012–5081, 64 p. (Available online at http://pubs.usgs.gov/sir/2012/5081/.)
iii

Acknowledgments

The authors thank the cooperators, Northern Shenandoah Valley Regional Commission and 

Central Shenandoah Planning District and localities therein, for support of this scientific work. 

Virginia Commonwealth University contributed equipment, travel funds, and supplies as well as 

in-kind services for this cooperative investigation to develop the fish habitat-suitability criteria. 

The continued interest and feedback received from policymakers, planners, utility managers, 

State agency staff, and private citizens enhanced the study process. 

Appreciation is extended to local landowners for allowing access to the river from their 

properties. John Gibson of the Downriver Canoe Company allowed access to the campgrounds 

that accommodated the field staff. Vital field assistance was provided by our lead technician 

for the project, Dennis Adams (USGS, Marion, Virginia) as well as volunteers from nonprofit 

organizations, State agencies, universities, and private citizens. Thanks to field volunteers: 

Jeff Kelble (Shenandoah River Keeper), Matt Balazik (Virginia Commonwealth University), 

Charlie Simmons (Virginia Department of Environmental Quality), Jon Campbell (U.S. Geological 

Survey, Reston, Virginia), and private citizen volunteers: Julie Ramey, Gene Ramey, Chet Ramey, 

and Frederic Bogar. The authors thank the peer reviewers who increased the effectiveness of 

this document.
iv
v

Contents

Acknowledgments........................................................................................................................................iii

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................2

Purpose and Scope ..............................................................................................................................3

Study Area..............................................................................................................................................3

Analysis of Historic Streamflow.........................................................................................................5

Water Withdrawals .....................................................................................................................8

Mesohabitat Summary of the South Fork Shenandoah River..............................................................10

Hydraulic Data Collection in Predominant Mesohabitat Types ..................................................12

Study Reach Selection and Description of Transects ..................................................................12

Hydraulic Data-Collection Methods ................................................................................................16

Fish-Community Sampling and Microhabitat Observations .................................................................16

Data-Collection Methods ..................................................................................................................16

Development and Testing of Fish Habitat Suitability Criteria ......................................................18

Canoeing Suitability Criteria ....................................................................................................19

RHABSIM Modeling to Determine Fish Habitat Availability and Recreation Conditions.................20

Model Calibration................................................................................................................................20

Water-Surface-Level Calibration ............................................................................................20

Velocity Calibration....................................................................................................................22

Habitat Simulation and Development of Weighted Usable-Habitat Area Curves....................23

Habitat-Discharge Relations for Streamflow-Gaging Stations on the South Fork .........23

Lynnwood Ecological Habitat .........................................................................................23

Canoeing through the Lynnwood Section............................................................30

Luray Ecological Habitat..................................................................................................30

Canoeing through the Luray Section....................................................................30

Front Royal Ecological Habitat .......................................................................................30

Canoeing through the Front Royal Section..........................................................33

Habitat Time-Series Scenario Analysis for Low-Flow Periods ...................................................38

Upper Section Time-Series Scenarios...................................................................................38

Middle Section Time-Series Scenarios .................................................................................41

Lower Section Time-Series Scenarios...................................................................................41

Water-Withdrawal Reduction Scenario Results ..................................................................46

Increased Water-Use Scenario Results ................................................................................46

Summary and Conclusions.........................................................................................................................54

References....................................................................................................................................................56

Appendix 1. Graphs showing fish habitat suitability criteria for depth, velocity, and 

 substrate characteristics on the South Fork Shenandoah River, Virginia......... 60–61 

Appendix 2. Table listing river habitat simulation model-calibration data from the 

 hydraulic data-collection reaches on the South Fork Shenandoah 

 River, Virginia ............................................................................................................... 62–63
vi

Figures

1. Map of South Fork Shenandoah River, Virginia, showing watershed and study sites .....4

2–4. Graphs showing number of days per month and year with streamflow values less

than or equal to the 10th percentile flow for July, August, and September for the:

2. Lynnwood streamflow-gaging station on the South Fork Shenandoah 

 River, Virginia ............................................................................................................................7

3. Luray streamflow-gaging station on the South Fork Shenandoah River, 

 Virginia........................................................................................................................................7

4. Front Royal streamflow-gaging station on the South Fork Shenandoah 

 River, Virginia ............................................................................................................................7

5. Map showing surface-water and groundwater withdrawal locations for the 

Shenandoah River watershed in Virginia, 2005 .......................................................................9

6–9. Photographs showing transect locations at:

6. Lynnwood on the South Fork of the Shenandoah River, Virginia ...............................13

7. Riverbend Pool on the South Fork of the Shenandoah River, Virginia ......................14

8. Kauffman Mill on the South Fork of the Shenandoah River, Virginia ........................15

9. Thunderbird Farms on the South Fork of the Shenandoah River, Virginia...............15

10. Graphs showing average depth and average velocity data input to student’s 

T–test of occupied and unoccupied habitats for each species of the 

South Fork Shenandoah River, Virginia...................................................................................20

11–13. Graphs showing weighted usable-habitat area of ecological and recreational 

habitat:

11. Combined for the Lynnwood and Riverbend Pool study reaches on the 

 South Fork of the Shenandoah River, Virginia................................................................ 27

12. At the Lynnwood study reach on the South Fork of the Shenandoah 

 River, Virginia ......................................................................................................................... 28

13. At the Riverbend Pool study reach on the South Fork of the Shenandoah 

 River, Virginia ......................................................................................................................... 29

14. Graphs showing canoeing suitability for selected transects at the Lynnwood 

study reach on the South Fork of the Shenandoah River, Virginia.....................................31

15. Graph showing weighted usable-habitat area of ecological and recreational 

habitat at the Kauffman Mill study reach on the South Fork of the Shenandoah 

River, Virginia...............................................................................................................................33

16. Model grid of canoeing depth suitability for the Kauffman Mill study reach 

on the South Fork of the Shenandoah River, Virginia ...........................................................34

17. Graph showing weighted usable-habitat area of ecological and recreational 

habitat at Thunderbird Farms in the lower reach on the South Fork 

Shenandoah River, Virginia.......................................................................................................36

18. Graphs showing canoeing suitability for the bedrock riffle at transect 4 at the 

Thunderbird Farms study reach on the South Fork Shenandoah River, Virginia .............37

19–24. Graphs showing habitat time series for the:

19. Upper section of the South Fork Shenandoah River near Lynnwood, 

 Virginia, during 2002............................................................................................................. 39

20. Upper section of the South Fork Shenandoah River near Lynnwood, 

 Virginia, during 1977 ............................................................................................................. 40

21. Middle section of the South Fork Shenandoah River near Luray, 

 Virginia, during 2002............................................................................................................. 42
vii

22. Middle section of the South Fork Shenandoah River near Luray, 

 Virginia, during 1999 ............................................................................................................. 43

23. Lower section of the South Fork Shenandoah River near Front Royal, 

 Virginia, during 2002............................................................................................................. 44

24. Lower section of the South Fork Shenandoah River near Front Royal, 

 Virginia, during 1966 ............................................................................................................. 45

25–27. Graphs showing habitat time-series water-withdrawal reduction scenarios

for the:

25. Upper section of the South Fork Shenandoah River near Lynnwood, 

 Virginia, during 2002............................................................................................................. 47

26. Middle section of the South Fork Shenandoah River near Luray, 

 Virginia, during 2002............................................................................................................. 48

27. Lower section of the South Fork Shenandoah River near Front Royal, 

 Virginia, during 2002............................................................................................................. 49

28. Graphs showing canoeing habitat time-series water-withdrawal reduction 

scenarios and habitat time-series increased water-withdrawal scenarios 

for the upper section of the South Fork Shenandoah River near Lynnwood, 

Virginia, during 2002 ...................................................................................................................50

29–31. Graphs showing habitat time-series increased water-withdrawal scenarios

for the:

29. Upper section of the South Fork Shenandoah River near Lynnwood, 

 Virginia, during 2002..............................................................................................................51

30. Middle section of the South Fork Shenandoah River near Luray, 

 Virginia, during 2002............................................................................................................. 52

31. Lower section of the South Fork Shenandoah River near Front Royal, 

 Virginia, during 2002............................................................................................................. 53

Tables

1. Selected streamflow-gaging stations in the Shenandoah River Basin, Virginia .......................5

2. Streamflow statistics for gages on the South Fork Shenandoah River, Virginia .......................6

3. Surface-water and groundwater withdrawals for 2005 in the Shenandoah River 

 watershed, and cumulative withdrawals for the South Fork Shenandoah River, 

 Virginia....................................................................................................................................................8

4. Results of the mesohabitat classification and mapping for three sections of the 

 South Fork Shenandoah River, Virginia...........................................................................................11

5. Locations of river reaches studied during hydraulic data collection and fish 

 habitat-suitability criteria development on the South Fork Shenandoah River, 

 Virginia..................................................................................................................................................12

6. Mesohabitat types represented by transects in the hydraulic data-collection 

 reaches on the South Fork Shenandoah River, Virginia...............................................................13

7. Best-estimate discharges used in the River Habitat Simulation model for each 

 hydraulic data-collection reach on the South Fork Shenandoah River, Virginia.....................16

8. Substrate codes used to classify the dominant and subdominant substrate within 

 fish study reaches and hydraulic data-collection reaches on the South Fork 

 Shenandoah River, Virginia ...............................................................................................................17

9. Codes used to classify the cover found around fish observations and within 

 hydraulic data-collection reaches on the South Fork Shenandoah River, Virginia.................18
viii

10. Suitable range of depths for fish in the South Fork Shenandoah River, Virginia.....................18

11. Suitable range of velocity for fish in the South Fork Shenandoah River, Virginia...................19

12. Suitable range of channel index for fish in the South Fork Shenandoah River, 

 Virginia..................................................................................................................................................19

13. Water-surface-level calibration methods and velocity calibration discharges used 

 in the River Habitat Simulation model for each hydraulic data-collection reach and 

 transect on the South Fork Shenandoah River, Virginia...............................................................21

14. Range of discharges used in the River Habitat Simulation model for simulations of 

 water-surface levels, velocities, and habitats on the South Fork Shenandoah 

 River, Virginia.......................................................................................................................................22

15. Weighted usable-habitat area for the upper section, including the 

 Lynnwood study reach and the Riverbend Pool study reach on the South Fork 

 Shenandoah River, Virginia ...............................................................................................................24

16. Weighted usable-habitat area for the Riverbend Pool study reach on the South 

 Fork Shenandoah River, Virginia ......................................................................................................25

17. Weighted usable-habitat area for the Lynwood study reach on the South Fork 

 Shenandoah River, Virginia ...............................................................................................................26

18. Weighted usable-habitat area for the Kauffman Mill study reach on the South Fork

 Shenandoah River, Virginia ...............................................................................................................32

19. Weighted usable-habitat area for the Thunderbird Farms study reach on the 

 South Fork Shenandoah River, Virginia...........................................................................................35
ix

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 2.590 square kilometer (km2) 

Flow rate

foot per second (ft/s) 0.3048 meter per second (m/s)

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

foot per mile (ft/mi) 0.1894 meter per mile (m/km)

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3/s)

Vertical coordinate information is referenced to the North American Vertical 

Datum of 1988 (NAVD 88).

Horizontal coordinate information is referenced to the North American 

Datum of 1983 (NAD 83).

Acronyms and Abbreviations

ADCP Acoustic Doppler Current Profiler

GPS global positioning system

HABSIM Habitat Simulation model 

HSC habitat-suitability criteria

JAS July, August, September low-flow period. Signifies statistics 

 were calculated only based on data for those 3 months 

 over the period of record

PHABSIM Physical Habitat Simulation model

RHABSIM River Habitat Simulation model 

USGS U.S. Geological Survey

VAF velocity adjustment factor

WSL water-surface level

WUA weighted usable-habitat area (per 1,000 feet of stream)
x
South Fork Shenandoah River Habitat-Flow 

Modeling to Determine Ecological and 

Recreational Characteristics during 

Low-Flow Periods

By Jennifer L. Krstolic1 and R. Clay Ramey 2

Abstract

The ecological habitat requirements of aquatic organisms 

and recreational streamflow requirements of the South Fork 

Shenandoah River were investigated by the U.S. Geological 

Survey in cooperation with the Central Shenandoah Valley 

Planning District Commission, the Northern Shenandoah 

Valley Regional Commission, and Virginia Commonwealth 

University. Physical habitat simulation modeling was 

conducted to examine flow as a major determinant of physical 

habitat availability and recreation suitability using fieldcollected hydraulic habitat variables such as water depth, 

water velocity, and substrate characteristics. Fish habitatsuitability criteria specific to the South Fork Shenandoah 

River were developed for sub-adult and adult smallmouth 

bass (Micropterus dolomieu), juvenile and sub-adult 

redbreast sunfish (Lepomis auritus), spotfin or satinfin shiner 

(Cyprinella spp.), margined madtom (Noturus insignis), and 

river chub (Nocomis micropogon). Historic streamflow statistics for the summer low-flow period during July, August, and 

September were used as benchmark low-flow conditions and 

compared to habitat simulation results and water-withdrawal 

scenarios based on 2005 withdrawal data. 

1U.S. Geological Survey, Richmond, Virginia.

2Virginia Commonwealth University, Richmond, Virginia, and 

ETI Professionals, Denver, Colorado.

To examine habitat and recreation characteristics during 

droughts, daily fish habitat or recreation suitability values 

were simulated for 2002 and other selected drought years. 

Recreation suitability during droughts was extremely low, 

because the modeling demonstrated that suitable conditions 

occur when the streamflows are greater than the 50th percentile 

flow for July, August, and September. Habitat availability 

for fish is generally at a maximum when streamflows are 

between the 75th and 25th percentile flows for July, August, 

and September. Time-series results for drought years, such as 

2002, showed that extreme low-flow conditions less than the 

5th percentile of flow for July, August, and September corresponded to below-normal habitat availability for both game 

and nongame fish in the upper section of the river. For the 

middle section near Luray, margined madtom and river chub 

habitat area were below normal, whereas adult and sub-adult 

smallmouth bass habitat area remained near the median 

expected available habitat. In the lower section near Front 

Royal, time-series results for adult smallmouth bass, sub-adult 

smallmouth bass, and margined madtom habitat were below 

normal when streamflows were below the 10th percentile flow 

for July, August, and September. All other species of fish had 

habitat availability within the normal range for July, August, 

and September. 

Water-conservation scenarios representing a 50‐percent 

water-withdrawal reduction resulted in game fish habitat 

availability within the normal range for habitat in upper and 

middle river sections, instead of below normal conditions 

which were observed during the 2002 drought. The 50‐percent 

water-withdrawal reduction had no measurable effect on 
2 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

recreation. For nongame fish such as river chub, a 20‐percent 

withdrawal reduction resulted in habitat availability within the 

normal range for habitat in the upper and middle river sections. 

Increased water-use scenarios representing a 5‐percent 

increase in water withdrawals resulted in a slight reduction in 

habitat availability; however, increased withdrawals of 20 and 

50 percent resulted in habitat availability substantially less than 

the 25th habitat percentile, or below normal. Habitat reductions were more pronounced when flows were lower than the 

10th percentile flow for July, August, and September. 

The results show that for normal or wet years, increased 

water withdrawals are not likely to correspond with extensive 

habitat loss for game fish or nongame fish. During drought 

years, however, a 20‐ to 50‐percent increase in water withdrawals may result in below normal habitat availability 

for game fish throughout the river and nongame fish in the 

upper and middle sections of the river. These simulations of 

rare historic drought conditions, such as those observed in 

2002, serve as a baseline for development of ecological flow 

thresholds for drought planning. 

Introduction

The South Fork Shenandoah River (referred to as the 

South Fork in this report), and its counterpart, the North Fork 

Shenandoah River (referred to as the North Fork), join to 

form the Shenandoah River, which drains an area that many 

in Virginia refer to as “the Valley.” With Shenandoah National 

Park to the east, and Massanutten Mountain to the west, the 

South Fork Basin is an area with much beauty, attracting new 

visitors and residents, many of whom are outdoor enthusiasts, 

tourists, and farmers. As population growth continues and new 

industries increase in the Valley, competition for clean water is 

a concern for policy makers, managers, planners, and citizens 

in the area who recognize the need to monitor and protect 

the South Fork streamflow as a resource for water supply, 

recreation such as canoeing, and ecological habitat for aquatic 

life. In 2005, the U.S. Geological Survey (USGS), in cooperation with the Central Shenandoah Valley Planning District 

Commission, the Northern Shenandoah Valley Regional 

Commission, and Virginia Commonwealth University, began 

an investigation to examine the instream flow needs of aquatic 

organisms of the South Fork as a companion study to the 

instream flow study on the North Fork (Krstolic and others 

2006). As a result of the current investigation, the counties and 

communities in the South Fork Basin should gain knowledge 

of the water resources in the basin, the availability of water for 

fish habitat and canoeing, and the potential effects of withdrawals and conservation measures on the ecology, recreation, 

agriculture, industry, and water supply.

The study design was patterned after the North Fork 

study to provide consistent modeling output for Valley planners and water-resource managers. Flow data and physical 

habitat data were collected at permanent cross sections 

representing habitat types in the South Fork. These flow data 

were input to the River Habitat Simulation Software model 

(RHABSIM), a 1‐dimensional water-surface-profile model 

that uses stage-discharge ratings to simulate habitat conditions over a range of streamflows. The modeling was used to 

determine the relation between suitable habitat and streamflow. This report completes the modeling for the two major 

tributaries of the Shenandoah River, providing consistent 

results to planning district personnel toward effective drought 

preparation in the planning districts that span both watersheds. 

The use of RHABSIM (or the original Physical Habitat 

Simulation model; PHABSIM) has been widespread 

throughout the past 30 years as part of the instream-flow 

incremental methodology approach to river management 

(Stalnaker, 1979). In fact, RHABSIM techniques for 

water-resources management have been used in the decision making process in more than 20 countries, because the 

model incorporates ecological variables toward realistic 

outcomes (Tharme, 2003; Petts, 2009). The instream-flow 

incremental methodology approach to river management can 

use RHABSIM to explicitly link physical habitat simulation 

with habitat-suitability criteria for species and life stages 

to display the relations between habitat usability and flow 

(Petts, 2009). The method relies on three principles that are 

important to this investigation: (1) the chosen species exhibits 

preferences within a range of habitat conditions that it can 

tolerate, (2) these ranges can be defined for each species or 

life stage, and (3) the area of stream providing these conditions can be quantified as a function of discharge and channel 

structure (Petts, 2009). Other modeling and data-collection 

platforms were considered as the study was being designed 

in 2005. A two-dimensional modeling approach was considered; however, the shallow, wide dimensions of the South 

Fork limited the acoustic velocity instrumentation that was 

available. Instruments that function in depths shallower than 

2 feet (ft) were not coupled with global positioning system 

(GPS) technology to spatially quantify the velocity fields. 

Because of the need to collect spatially relevant velocity data, 

traditional RHABSIM data-collection methods were selected. 

The association between flow variability and the health 

of river ecosystems is not a simple relation, because the 

linkages are complex in both time and space (Petts, 2009). 

The complexity of the relation has brought about criticism 

for the instream-flow incremental methodology approach 

for water-resources management. This is especially true 

when minimum flows are developed for a single species, 

according to Annear and others (2002), who emphasize that 

a single flow value (minimum, optimal, or otherwise) cannot 

simultaneously meet the requirements for all species or 

maintain a fishery. This investigation was begun under specific 

assumptions about fish habitat use during low-flow conditions, so findings are only applicable to that part of the flow 

regime. The applicability of the RHABSIM modeling results 
Introduction 3

for high-flow periods or those related to reproduction or 

other physiological functions was not evaluated. A variety of 

species and life stages of both game and nongame warm-water 

fishes were evaluated for development of habitat-suitability 

criteria (HSC) so that modeling results would be representative of a broad range of species habitat needs. Ideally, the 

fish-community population size and age distribution would be 

monitored throughout the duration of the study, or a longterm fish community dataset would be available for all game 

and nongame species and linked with streamflow statistics. 

Neither option was entirely possible given study-scope 

limitations. However, a thorough evaluation of fish habitat use 

during the low-flow period was completed during June 2008 

through September 2008. The flows during that time period 

were between the 50th and 75th historic monthly percentiles 

in June and September but were between the 10th and 25th

historic monthly percentiles for July and August. These flows 

represent normal flows and below-normal flows, respectively. 

A key assumption of this investigation and many others 

is that flow is a major determinant of physical habitat in rivers, 

which in turn is a major determinant of biotic composition 

(Bunn and Arthington, 2002). Another assumption is that 

physical habitat is a limiting factor for fish populations. 

Other factors that can determine whether or not fish occupancy increases or decreases with physical habitat availability 

are water quality, predation, competition, food availability, 

and disease. If other factors that were not studied on the 

South Fork have a greater effect on population success, 

then managing water supply for habitat may not promote 

populations. It should be emphasized that the key variables 

being evaluated by the RHABSIM model are components of 

hydraulic habitat (water depths, water velocities, and substrate 

characteristics) and discharge. For a study that is focused on 

water availability and habitat availability during drought, the 

approach represents a surrogate for biological response fundamental to an organism’s existence (Annear and others, 2002). 

This approach is powerful, because it links fish habitat availability (in this case) to discharge (Annear and others, 2002) 

and water withdrawals. Discharge and water withdrawals 

are the variables that are commonly used by water-resource 

managers and planners, thereby making modeling results 

more easily applied. No dam building or human engineering 

and alteration of the channel form, excluding those occurring 

with natural floods, have occurred in the past 20 to 30 years. 

Therefore, an assumption can be made that no net changes 

due to physical alteration of habitat are occurring on the 

South Fork. The results of the RHABSIM modeling were 

used in time-series scenarios that predict habitat duration or 

availability typical of historic and drought streamflow conditions (1930–2008) in the South Fork. These scenarios should 

assist managers in selecting future thresholds for discharge 

and habitat that are protective of aquatic organisms and that 

provide water supply for human and industrial use in the South 

Fork Shenandoah River watershed. 

Purpose and Scope

The purpose of this report is to describe the methods, 

document results, and discuss implications of flow habitat 

modeling for the South Fork Shenandoah River in Virginia. 

A range of scenarios is presented to provide managers and 

planners with information regarding current and future 

water resources in the basin, the availability of water for fish 

habitat, recreation, and the potential effects of withdrawals 

and conservation measures on fish populations in the South 

Fork. The objectives of this investigation are to enhance the 

understanding of summer low-flow conditions in the South 

Fork relative to the physical habitat needs of fish, and to 

analyze water use and recreation needs of humans, such as 

canoeing conditions along certain stream riffles. Specifically, 

study objectives were to incorporate mesoscale and microscale 

physical habitat information and detailed streamflow data into 

1‐dimensional physical habitat models to quantify availability 

of suitable fish habitat during low-flow conditions, as well as to 

develop water-use and water-conservation scenarios for current 

and future water withdrawals that might affect aquatic habitat.

Study Area

The South Fork Shenandoah River and its watershed are 

the main focus of this investigation. The watershed begins in 

Augusta County, Virginia (Va.) by three main tributaries—the 

North, Middle, and South Rivers—which merge to form the 

South Fork near Lynnwood, Va. The South Fork flows north 

97 miles (mi) to meet the North Fork Shenandoah River at 

the Town of Front Royal (Virginia Department of Game and 

Inland Fisheries, 2011) (fig. 1). The South Fork is a typical 

low-gradient stream with a main channel slope that ranges from 

3.84 to 0.55 feet per mile (ft/mi) from upstream to downstream 

(Austin and others, 2011), and has an average basin slope 

that ranges from 290 to 306 ft/mi (Paybins, 2008; Austin and 

others, 2011). Average basin slope is measured by summing the 

length of all elevation contours in miles, multiplied by the 20‐ft 

contour interval, then divided by the square mileage of the 

drainage area (Harvey and Eash, 1996; Paybins, 2008; Austin 

and others, 2011). The South Fork is a typical low-gradient 

stream with a main channel slope that ranges from 3.84 to 

0.55 ft/mi from upstream to downstream (Austin and others, 

2011), and has an average basin slope that ranges from 290 to 

306 ft/mi (Paybins, 2008; Austin and others, 2011), but it does 

have some Class I and Class II rapids (Virginia Department of 

Game and Inland Fisheries, 2011). The 1,634‐square mile (mi2) 

watershed generally is long and narrow, as it is confined over 

most of its length by the Blue Ridge Mountains to the east and 

Massanutten Mountain to the west. 

Land use in the South Fork watershed above Front Royal 

is primarily forest (55 percent), grass or pasture (30 percent), 

urban (11 percent), and row-crop agriculture (4 percent) (MultiResolution Land Characteristics Consortium, 2001). The geology 
4 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

Elevation

 1,359 meters

 67 meters

River Management sections

 Upper

 Middle

 Lower

 Major watersheds

Streamflow-gaging station 

Study sites

 Fish only

 South Fork study reach

City

Staunton

Grottoes

Port Republic

Waynesboro

Shenandoah

Winchester

Front Royal

Harrisonburg

Harpers Ferry

Strasburg

Mount Jackson

78°0'

77°0'

39°0'

38°0'

ROCKINGHAM

PAGE

FREDERICK

SHENANDOAH

BERKELEY

WARREN

JEFFERSON

CLARKE

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Map credit on following page.

Riverbend Pool

Kauffman Mill

Route 661

Thunderbird Farms

Compton Pool

Lynnwood

Front Royal Gage

Luray Gage

Lynnwood Gage

 South Fork Shenandoah River

North Fork Shenandoah River

Shenandoah River

North River

Middle River

South River

Cootes Store Gage

Newport

AUGUSTA

WEST

VIRGINIA

WEST

VIRGINIA

VIRGINIA

VIRGINIA

Fig. 1
Introduction 5

of the total upstream area of the watershed at Front Royal 

also is representative of the watershed segments upstream 

at Luray or Lynnwood. The geology consists of Blue Ridge 

metamorphic rock (10 percent), Valley and Ridge carbonate 

rock (48 percent), and Valley and Ridge silicilastic rock 

(41 percent) (Virginia Department of Mines, Minerals, and 

Energy; Division of Mineral Resources, 2005; Austin and 

others, 2011). The valley bottoms are made up of sandstone, shale, and carbonate rocks (Yager and others, 2008). 

The ridges and hill slopes within the basin are primarily 

underlain by silicilastic rocks. The southern end of the basin 

adjacent to the Blue Ridge generally is referred to as the 

“western-toe carbonate unit,” which is made up of carbonate 

rocks overlain by colluvial gravel (Yager and others, 2008). 

This area has depths to bedrock that are greater than other 

places in the Shenandoah River watershed and is an important 

recharge area for groundwater (Morgan and others, 2004; 

Swain and others, 2004; Yager and others, 2008).

Using the National Hydrography Dataset for reporting 

study results, the South Fork, beginning near Port Republic, 

was divided into upper (0 to 25 river miles), middle (26 to 55 

river miles), and lower (56 to 106 river miles) sections (fig. 1). 

Each of these sections ends at a hydrogeomorphic discontinuity (Larned and others, 2010), either manmade impoundments or a river confluence. Streamflow-gaging stations are 

located within each section: South Fork Shenandoah River 

near Lynnwood (01628500), South Fork Shenandoah River 

near Luray (01629500), and South Fork Shenandoah River at 

Front Royal (01631000) (fig. 1, table 1). 

Analysis of Historic Streamflow

Streamflows of the South Fork have been monitored for 

at least 79 years by the USGS and the Virginia Department 

of Environmental Quality. The long-term records from three 

streamflow-gaging stations (table 1) can be used to characterize the flow regime (seasonal, monthly, and annual patterns) 

of the South Fork that may affect water availability and habitat 

for aquatic species. The period of record for the Front Royal 

and Lynnwood streamflow-gaging stations include data from 

1931 and 1930, respectively, to the present. A continuous 

record for the Luray streamflow-gaging station is only available from 1925 to 1951 and from 1979 to the present (only the 

more recent period at the Luray gage was used in this study). 

Records from these three streamflow-gaging stations were 

used to represent the variation in statistics longitudinally as 

the drainage area increases. Long-term streamflow data from 

these three stations provide an opportunity to characterize 

dry, wet, and normal years, and subsequently, to assess fish 

habitat availability during these contrasting conditions. Water 

is withdrawn for a variety of uses from the South Fork as well 

as three power-supply dams that represent nonconsumptive 

water use. During extreme low-flow periods, dam releases 

may be evident in the streamflow record; however, the dams 

are generally flow-over spillways that do not commonly alter 

the natural flow regime of the river. 

Figure 1. At left—South Fork Shenandoah River, Virginia, watershed and study sites. 

 Map credit—

Elevation from U.S. Geological Survey National Elevation Data Set digital elevation model, 2005, 10-meter resolution

Universal Transverse Mercator 17 projection, NAD 83, central meridian -81 00’W, rotated 30 degrees

Hydrography from U. S. Geological Survey National Hydrography Dataset, accessed 2007, 1:24,000;

City location from U. S. Census Cartographic Boundary Files, Consolidated Cities, 2000, ~ 1:500,000

Watershed boundaries from Krstolic, 2007, Drainage basin delineations for selected USGS 

streamflow-gaging stations in Virginia 

Table 1. Selected streamflow-gaging stations in the Shenandoah River Basin, Virginia.

[SF, South Fork]

Station 

number Station name

Drainage 

area, 

square miles

Operating agency Period of 

record

01628500 SF Shenandoah River near Lynnwood 1,079 Virginia Department of Environmental Quality 1930–2008

01629500 SF Shenandoah River near Luray 1,372 Virginia Department of Environmental Quality 1979–2008

01631000 SF Shenandoah River at Front Royal 1,634 U.S. Geological Survey 1931–2008
6 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

To identify critical flow components, the beginning of 

any hydrologic assessment includes characterization of flow 

time series (Larned and others, 2010), including the development of flow-duration curves, which is a summarization of 

streamflows grouped into percentiles. These percentiles are 

based on a scale of 1 to 100, and indicate the percentage of 

a data distribution that is equal to or below each percentile 

value. For example, the 90th percentile of a river is equal 

to or greater than 90 percent of the daily discharge values 

recorded during all years that measurements have been made. 

A discharge at the 10th percentile is equal to or greater than 

10 percent of recorded daily values, making it represent the 

lower end of flows. In this report, the percentiles are based on 

historic daily values calculated for a given time period, which 

are typically annual records or the low-flow summer months 

of July, August, and September (JAS). In the recent publication of the North Fork Shenandoah River (Krstolic and others, 

2006) a variation of the percentile known as the “percent 

exceedance” was used. Percent exceedance (Searcy, 1969) 

is obtained by subtracting the percentile scale value from 

100 percent. 

 In this document, most flow units are referred to in 

cubic feet per second (ft3/s), but tables and figures include 

million gallons per day (Mgal/d) where relevant (1 ft3/s 

equals 0.6465 Mgal/d). This study primarily incorporates 

flow percentiles calculated on an annual basis and calculated 

over the summer low-flow period during JAS (table 2). 

For reference, the report uses the 25–75th percentile range as 

a “normal” condition. Table 2 shows the 25–75th percentile 

range for each streamflow-gaging station. Flows less than 

the 25th percentile represent conditions that indicate less 

water availability and may indicate drought as the percentiles 

decrease. As in the North Fork investigation (Krstolic and 

others, 2006), historic data from JAS are used to assess flows 

during the time of year when precipitation is low, temperatures 

are high, and water-use demands are high. 

Years having flows much lower than the normal range 

of flows in JAS (typically 10th percentile JAS flow or lower) 

were classified as dry years (Krstolic and others, 2006). 

Typically, a 10th percentile JAS flow occurs 3 days per month; 

therefore, when more than 50 percent of days in a given 

month have a discharge below this threshold, it is an indicator 

that water availability is scarce. Figures 2, 3, and 4 illustrate 

the numbers of days per month flow decreased below the 

10thpercentile JAS flow during the period of record for each 

streamflow-gaging station. As a conservative measure to 

characterize drought years, the 10th percentile annual flow was 

considered. For the South Fork, the 10th percentile annual flow 

occurred an average of 69 to 96 percent of the days during 

JAS during drought years. 

Table 2. Streamflow statistics for gages on the South Fork Shenandoah River, Virginia. 

[Annual statistics represent conditions over all months of the year for the entire period of record for each gage through 

2008. July–August–September statistics represent the 3-month average flow conditions for the entire period of record 

through 2008. Monthly statistics are updated daily and available for each streamflow-gaging station at http://va.water.

usgs.gov/duration_plots/dp_map_potomac.htm]

Percentile

Lynnwood 

01628500

Luray 

01629500

Front Royal 

01631000

Lynnwood 

01628500

Luray 

01629500 

Front Royal 

01631000

Million gallons per day Cubic feet per second

Annual

95 2,023 2,702 3,005 3,130 4,180 4,650

90 1,357 1,790 2,042 2,100 2,770 3,160

75 724 976 1,138 1,120 1,510 1,760

50 390 535 613 604 828 948

25 218 304 344 337 470 533

10 152 228 249 235 352 386

5 128 198 212 198 307 328

July–August–September

95 1,034 1,480 1,525 2,290 2,290 2,360

90 633 737 963 1,140 1,140 1,490

75 352 497 537 769 769 831

50 233 317 360 491 491 557

25 171 246 271 380 380 420

10 131 200 213 310 310 330

5 115 169 187 262 262 290
Introduction 7

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

1930193219341936193819401942194419461948195019521954195619581960196219641966196819701972197419761978198019821984198619881990199219941996199820002002200420062008

NUMBER OF DAYS

YEAR

July

August

September

EXPLANATION

Figure 2. Number of days per month and 

year (1930–2008) with streamflow values 

less than or equal to the 10th percentile 

flow for July, August, and September for 

the Lynnwood streamflow-gaging station 

(01628500) on the South Fork Shenandoah 

River, Virginia.

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

NUMBER OF DAYS

YEAR

July

August

September

EXPLANATION

197919811983198519871989199119931995199719992001200320052007

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

1930193219341936193819401942194419461948195019521954195619581960196219641966196819701972197419761978198019821984198619881990199219941996199820002002200420062008

NUMBER OF DAYS

YEAR

July

August

September

EXPLANATION

Figure 3. Number of days per month and 

year (1979–2008) with streamflow values 

less than or equal to the 10th percentile 

flow for July, August, and September 

for the Luray streamflow-gaging station 

(01629500) on the South Fork Shenandoah 

River, Virginia.

Figure 4. Number of days per month and 

year (1930–2008) with streamflow values 

less than or equal to 10th percentile flow 

for July, August, and September for the 

Front Royal streamflow-gaging station 

(01631000) on the South Fork Shenandoah 

River, Virginia.
8 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

Water Withdrawals

Water-use data for the South Fork were compiled from the 

most recent USGS publication available (Kenny and others, 2009). 

The data in this report are total withdrawals taken from the river 

and from groundwater for 2005, without accounting for water 

returns. Estimates of consumptive use compared to return flow 

are quite complicated and are beyond the scope of this investigation. Water-use categories in Kenny and others (2009) included 

location-specific withdrawal datasets, such as public-water supply, 

commercial industrial, thermoelectric, hydroelectric, golf course 

irrigation, and mining, as well as nonspatially referenced categories with water withdrawals summarized by county, including row 

crop irrigation, livestock, aquaculture, and domestic self-supply 

groundwater wells. The county-level data for row crops, livestock, 

aquaculture, and domestic self-supply wells were approximated, 

based on published methods for estimating water use (Lovelace, 

2009a–c). The only category that was excluded from this summary 

was hydroelectric power, because the majority of the water 

used was returned to the river. The location-specific water-use 

data were summarized for comparison purposes for each major 

watershed section of the South Fork in the study area, as well as 

for the tributaries upstream, the North Fork, and the mainstem 

Shenandoah River. The county-level data water-use estimates were 

apportioned to each watershed based on the percentage of county 

land area it contained and added to the location-specific water-use 

data (table 3). The categories of surface-water and groundwater 

withdrawals in the Shenandoah Valley are shown in figure 5.

Because water withdrawals from an upstream section 

could reduce water availability downstream, cumulative water 

withdrawals were calculated from upstream to downstream. 

For example, total water withdrawals for Luray are equal 

to the water withdrawals for the watershed around Luray in 

addition to the water withdrawals for the North, Middle, South 

River watersheds and the area draining to Lynnwood (fig. 5). 

The largest surface-water withdrawals were located in 

the tributaries to the South Fork in the North (17.7 Mgal/d), 

Middle (10.4 Mgal/d), and South (9.8 Mgal/d) Rivers, respectively (table 3). Much of the water-use estimates for Augusta 

County came from aquaculture withdrawals that equaled 

25.9 Mgal/d and were proportionally distributed across the 

Table 3. Surface-water and groundwater withdrawals for 2005 in the Shenandoah River watershed, and cumulative 

withdrawals for the South Fork Shenandoah River, Virginia.

[Hydroelectric power withdrawal values were omitted because they were assumed to be 100-percent returned flow. Data1 from Kenny and 

others, 2009. NF, North Fork; SF, South Fork]

Station 

number Name

Million gallons per day Cubic feet per second

Surface 

water

Groundwater Total Surface 

water

Groundwater Total

01632000 NF Shenandoah River at Cootes Store 3.3 1.1 4.3 5.0 1.7 6.7

01633000 NF Shenandoah River at Mount Jackson 7.5 2.6 10.1 11.6 4.0 15.6

NF Shenandoah at mouth 18.4 4.5 23.0 28.5 7.0 35.5

South River at mouth 9.8 12.3 22.1 15.1 19.0 34.1

Middle River at mouth 10.4 3.9 14.4 16.2 6.1 22.3

North River at mouth 17.7 3.9 21.7 27.4 6.1 33.6

01628500 SF Shenandoah River near Lynnwood 0.4 0.1 0.6 0.7 0.2 0.9

01629500 SF Shenandoah River near Luray 3.5 13.6 17.0 5.4 21.0 26.4

SF Shenandoah at mouth 2.4 1.7 4.0 3.6 2.6 6.2

Shenandoah River at mouth 3.5 1.1 4.6 5.5 1.6 7.1

Total 2005 water use in the Shenandoah River watershed 76.9 44.9 121.7 119.0 69.4 188.4

Cumulative withdrawals for the South Fork Shenandoah River

01628500 Lynnwood 38.3 20.3 58.7 59.4 31.5 90.9

01629500 Luray 41.8 33.9 75.8 64.8 52.5 117.3

01631000 Front Royal 44.5 35.6 79.8 68.4 55.1 123.5

1Location-specific withdrawal data including: public-water supply, commercial industrial, thermoelectric, hydroelectric, golf-course 

irrigation, and mining; and nonspatially referenced county-level data including: row-crop irrigation, livestock, aquaculture, and domestic 

self-supplied groundwater wells.
Introduction 9

AUGUSTA

ROCKINGHAM

PAGE

FREDERICK

SHENANDOAH

BERKELEY

WARREN

CLARKE

JEFFERSON

STAUNTON

WINCHESTER

HARRISONBURG

WAYNESBORO

SHENANDOAH

AUGUSTA

ROCKINGHAM

PAGE

STAUNTON

WAYNESBORO

WARREN

CLARKE

JEFFERSON

Base from Kenny and others, 2009 

Geographic projection, NAD 83, rotated 36 degrees;

Virginia Department of Conservataion and Recreation, 2004, 1:24,000;

U.S. Geological Survey National Atlas coverage, 1:2,000,000

78°0'

77°0'

79°0'

39°0'

38°0'

39°0'

Water withdrawals, 2005,

in million gallons per day

Groundwater + surface water

 0.6 – <5.0

 5.0 – <10.0 (not applicable)

 10.1 – <15.0

 15.0 – <20.0 

 20.0 – 23.0

Gaging stations

 Streamflow gaging station

Commercial industrial

 Surface water

 Groundwater

 Thermoelectric power

Golf course irrigation

 Surface water

 Groundwater

Mining

 Surface water

 Groundwater

Public supply

 Surface water

 Groundwater

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

WEST

VIRGINIA

WEST VIRGINIA

VIRGINIA

VIRGINIA

Figure 5. Surface-water and groundwater withdrawal locations for the Shenandoah River watershed 

in Virginia, 2005.
10 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

watershed by area. Discharge from these rivers forms the South 

Fork Shenandoah River. As the river flows to Lynnwood, an 

additional 0.4 Mgal/d is withdrawn; at Luray, an additional 

3.5 Mgal/d is withdrawn; and at the mouth of the South Fork, 

an additional 2.4 Mgal/d is withdrawn. The cumulative total 

surface-water withdrawal from the South Fork Shenandoah 

River was 44.5 Mgal/d (table 3), which does not account for 

water returns to the river. The annual median flow ranges 

from 390 to 612 Mgal/d for Lynnwood to Front Royal, and 

the summer JAS period has median flows ranging from 233 

to 360 Mgal/d. During the low-flow period, the surface-water 

withdrawals represent 12 to 19 percent of the median flow. 

Groundwater withdrawals are common in the South 

Fork watershed, and in a watershed that is underlain by a 

large amount of carbonate bedrock, the groundwater conditions are expected to be closely linked to stream conditions. 

A recent investigation in the Shenandoah River watershed 

found that source waters to tributary streams were 50‐ to 

80‐percent groundwater during dry conditions (Nelms and 

Moberg, 2010). Groundwater withdrawals are a substantial 

portion of the water used in the watershed. Groundwater 

withdrawals are highest in the watershed draining the 

South River (12.3 Mgal/d) and the watershed draining 

to Luray (13.6 Mgal/d), with the rest of the groundwater 

withdrawals coming from other parts of the South Fork 

watershed. The cumulative total for the South Fork watershed 

groundwater withdrawals is 35.6 Mgal/d (table 3). 

Mesohabitat Summary of the South 

Fork Shenandoah River

Mesohabitats are channel-spanning, moderately large 

hydrogeomorphic habitat units, such as a riffle, run, or pool 

having relatively homogeneous channel characteristics (Vadas, 

1992; Vadas and Orth, 1998; Krstolic and others, 2006). 

Mesohabitats for the entire South Fork were mapped during 

the summer in 2006 and 2007 following similar methods as 

the North Fork Shenandoah River (Krstolic and others, 2006) 

to gain an understanding of the habitat distribution during 

low-flow periods. Spatial data with habitat descriptor information for both rivers are available as a digital data release 

of Geographic Information Systems (GIS) data (Krstolic and 

Hayes, 2010). These data are useful for summarizing the 

overall habitat availability for fish in the river and serve as 

the basis for weighting of transect habitat types in the habitat 

modeling phase of this study. 

Mesohabitat characteristics for the South Fork are 

summarized in table 4 and follow the categories presented in 

Krstolic and others (2006), with the addition of the habitat 

category “glide,” to describe the downstream end of a pool 

where it shallows and transitions into riffle habitat (Krstolic 

and Hayes, 2010). Mesohabitat length throughout the South 

Fork consists of 52.6‐percent run, 35.7‐percent pool, and 

11.7‐percent riffle (table 4). 

Bedrock riffle and aquatic vegetation, South Fork Shenandoah River
Introduction 11

Table 4. Results of the mesohabitat classification and mapping for three sections of the South Fork Shenandoah River, Virginia.[ND, no data]

 Habitat type

Lynnwood (upper section) Luray (middle section) Front Royal (lower section) Whole River

Number

of 

sections

Length, 

miles

Percentage

of 

total

Percentage 

of total 

by 

habitat 

type

Median 

width, 

feet

Median 

average 

depth, 

feet

Number

of 

sections

Length, 

miles

Percentage

of 

total

Percentage 

of total 

by 

habitat 

type

Median 

width, 

feet

Median 

average 

depth, 

feet

Number

of 

sections

Length, 

miles

Percentage

of 

total

Percentage 

of total 

by 

habitat 

type

Median 

width, 

feet

Median 

average 

depth, 

feet

Number

of 

sections

Length, 

miles

Percentage

of 

total

Percentage 

of total 

by 

habitat 

type

Median 

width, 

feet

Median 

average 

depth, 

feet

Riffle Shallow rapids, with fast velocities, in an open stream where a turbulent water surface is created by obstructions that are wholly or partly submerged. Water depth is generally less than 1 foot. Substrate is either bedrock or boulder, cobble, gravel, and sand (particle).

Particle riffle 58 1.8

9.8

7.4 172.5 0.8 38 1.6

8.3

5.4 210.0 1.0 32 1.5

14.6

2.9 300.0 0.8 128 4.9

11.7

4.6 213.0 0.8

Bedrock riffle 21 0.6 2.4 264.0 0.8 34 0.9 2.9 286.5 1.0 129 6.0 11.7 336.0 0.8 184 7.5 7.1 306.0 0.8

Run

Swift moving areas characterized by predominantly smooth to slightly turbulent flow. The water surface is usually flat and is not broken by the substrate. Water depth is typically 1–5 feet. Gently sloping bed with fairly consistent and uniform depth. Pocket runs shallow areas of bedrock interspersed with a mosaic of deeper holes or “pockets” that provide cover for fish. Run substrate is either bedrock with little silt and sand, or cobble, gravel, and sand. 

Particle run 97 8.1

54.2

32.8 165.0 2.0 60 5.2

49.4

17.6 226.5 2.0 59 5.2

53.6

10.1 298.5 1.7 216 18.5

52.6

17.5 210.0 2.0

Bedrock run 61 4.9 20.0 207.0 2.3 92 9.0 30.3 252.0 2.3 213 21.3 41.6 297.0 2.0 366 35.3 33.4 273.0 2.0Pocket run 3 0.3 1.3 217.5 5.3 3 0.4 1.5 201.0 3.6 10 1.0 1.9 264.0 3.3 16 1.7 1.7 243.0 3.8

Pool

Areas with reduced or barely perceptible surface velocity, with a smooth, unbroken water surface. Water depth is greater than 5 feet. Natural pools are formed by a naturally occurring channel obstruction such as a bedrock control, island, large gravel bar, or a meander bend, whereas artificial pools are formed by obstructions such as dams. Glides are transitional habitats as a pool shallows into riffle or run habitat. 

Natural pool 60 5.6

36.0

23.0 213.0 6.0 56 4.7

42.3

15.9 249.0 5.0 105 11.5

31.8

22.5 289.5 5.0 221 21.9

35.7

20.7 255.0 5.5

Artificial pool 10 1.4 5.8 301.5 8.5 41 5.7 19.1 333.0 8.8 3 0.3 0.5 306.0 4.0 54 7.4 7.0 316.5 8.5Backwater 8 0.7 2.8 318.0 ND 11 1.1 3.7 120.0 ND 31 3.5 7.0 36.0 ND 50 5.3 5.1 105.0 NDGlide 22 1.1 4.4 207.0 2.0 19 1.1 3.6 243.0 2.1 17 0.9 1.8 333.0 2.5 58 3.1 2.9 243.0 2.0

Totals 340 24.5 100.0 100.0 354 29.7 100.0 100.0 599 51.3 100.0 100.0 1,293 105.6 100.0 100.0
12 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

The South Fork is a large river with an average wetted 

channel width of 260 ft during low flow that maintains similar 

channel form within each habitat type throughout most of its 

length. Bedrock run habitat makes up 33.4 percent of the river 

and is the most abundant of any habitat category (table 4). 

The bedrock in the South Fork is usually tilted or dipping, rather 

than having a flat-bottom surface, and can be described as a 

series of “ledges and trenches” that cross the river obliquely. 

As the alternating pattern of shallow to deep water in these 

areas makes canoeing or wading challenging, the pattern also 

provides a substantial amount of cover for fish, because they 

can hide in the deep, slower water between the bedrock ledges. 

A similar pattern was seen with bedrock runs and bedrock 

riffles. Bedrock substrates are least abundant upstream but 

increase in percentage downstream. Particle riffles and particle 

runs, however, are most abundant upstream and decrease in 

percentage downstream (table 4). 

Natural pools make up 20.7 percent of the length of the 

South Fork (table 4), the second most abundant habitat type. 

Average depths in natural pools are consistent throughout the 

river, ranging from 2.8‐ to 20.0‐ft deep, with a maximum depth 

of 35 ft. The median average depth in artificial pools is 8.5 ft, 

and the maximum depth is 18 ft (Krstolic and Hayes, 2010). 

Although bedrock riffles and particle riffles only make up 

11.7 percent of the overall habitat, they support many of the 

smaller nongame fish which serve as food for game fish. These 

areas offer faster water velocity, shallow depths, and diversity 

in substrate (gravel, cobble, boulder, and bedrock bottoms) 

where fish can hide and forage. Depths in bedrock riffles and 

particle riffles are consistent, with a median average depth of 

0.8 ft (Krstolic and Hayes, 2010). The mesohabitat data are 

used to prioritize hydraulic monitoring reaches and to weight 

modeling results to represent the habitats within the entire 

length of the South Fork. 

Table 5. Locations of river reaches studied during hydraulic data collection and fish habitat-suitability criteria development on the 

South Fork Shenandoah River, Virginia.

[Location information for the partial record station at each study reach. Reaches are presented in upstream to downstream order. DMS, degrees minutes seconds; 

NAD 83, North American Datum of 1983]

Station 

number Reach name Latitude DMS (NAD83)

Longitude DMS 

(NAD83) Station name

01628500 Lynnwood 38° 19′ 21′′ N 78° 45′ 18′′ W South Fork Shenandoah River near Lynnwood, Virginia

0162910630 Riverbend Pool 38° 27′ 29′′ N 78° 38′ 00′′ W South Fork Shenandoah River above Naked Creek near Shenandoah, Virginia

01629510 Kauffman Mill 38° 39′ 20′′ N 78° 32′′ 30′′ W South Fork Shenandoah River below Route 211 near Hamburg, Virginia

01630555 Route 6111 38° 44′ 57′′ N 78° 25′ 42′′ W South Fork Shenandoah River at Oak Hill, Virginia

01630600 Compton Pool1 38° 47′ 01′′ N 78° 22′ 34′′ W South Fork Shenandoah River above Dry Run at Compton, Virginia

01630790 Thunderbird Farms 38° 52′ 38′′ N 78° 15′ 26′′ W South Fork Shenandoah River below Gooney Run near Limeton, Virginia

1Only fish data were collected at these reaches.

Hydraulic Data Collection in Predominant 

Mesohabitat Types

Hydraulic data were used during model calibration 

and simulation to develop relations between water-surface 

levels (WSL) and discharge and between water velocity and 

discharge. Data were collected on the South Fork between 

May 2007 and August 2009. 

Study Reach Selection and Description of 

Transects

Study reaches were selected based on accessibility by boat 

or wading, variety of habitat, and distribution throughout the 

length of the South Fork. Study reaches are shown in figure 1 

and location information is presented in table 5. Study reaches 

span the length of the South Fork from near the confluence 

of the North, Middle, and South Rivers at Lynnwood to 

Thunderbird Farms near the confluence with the North Fork. 

Four reaches were selected for hydraulic data collection. All 

reaches contained at least one mesohabitat, and three to five 

semipermanent transects (following Bovee, 1997; table 6). 

Verticals used in velocity, depth, and substrate measurements 

were spaced at 10‐ft intervals along each transect (modified 

from Bovee, 1997).

The Lynnwood reach (figs. 1, 6), is the most upstream 

reach. A streamflow-gaging station is in operation at this site 

in line with transect 5, the most downstream transect. This 

reach has a smooth bedrock channel bottom with ledges that 

run parallel to flow. Parallel ledges are uncommon along most 

of the South Fork. Lynnwood also contains a particle riffle at 

transect 2 (table 6). Riverbend Pool, located upstream from 

the Town of Shenandoah (figs. 1, 7), is the next downstream 
Mesohabitat Summary of the South Fork Shenandoah River 13

Table 6. Mesohabitat types represented by transects in the hydraulic data-collection 

reaches on the South Fork Shenandoah River, Virginia.

[Transects are numbered in upstream to downstream order, beginning with number 0 or 1]

Reach name Transect Habitat type Reach name Transect Habitat type

Lynnwood 1 Run, bedrock Kauffman Mill 0 Glide

2 Riffle, particle 1 Riffle, particle

3 Run, bedrock 2 Run, particle

4 Run, bedrock 3 Run, bedrock

5 Run, bedrock 4 Run, bedrock

Riverbend Pool 1 Pool, bedrock Thunderbird 

Farms

1 Pool, bedrock

2 Pool, bedrock 2 Pool, bedrock

3 Pool, bedrock 3 Glide

4 Riffle, bedrock

Digital Orthophoto source: Virginia Geographic Information Network, 2007, VBMP Orthophotography—2006/2007 (VA State 

Plane North), 1:2,400 scale (1”= 200’), available online at http://www.isp.virginia.gov/vbmporthophotography.shtml

Figure 6. Transect locations at Lynnwood on the South Fork of the Shenandoah River, Virginia. 

Locations of study sites are shown on figure 1.

EXPLANATION

Transect location

 and number

Model-grid habitat

 sampling cell

Streamflow-gaging

 station and number

01628500

01628500

XS

1

2

3

4

5

5

 Model grid cells are 10 feet across.
14 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

reach. It is a natural pool with a control riffle downstream 

that is near the Shenandoah Dam Reservoir. Just upstream 

of Riverbend Pool is a high-gradient riffle at the point of a 

meander bend. The upstream transect at Riverbend Pool is a 

transitional transect, but slow and deep enough to be classified as bedrock pool. The two downstream cross sections 

(table 6) represent deeper pool habitats on the South Fork. 

These had slow velocities that averaged 0.15 foot per second 

(ft/s) during median low flows and less than 1.5 ft/s for the 

highest flows that were measured. The Kauffman Mill reach 

has the most diversity in habitat by having glide, particle 

riffle, particle run, and bedrock run transects. Kauffman Mill 

is located about 1 mi downstream from the streamflow-gaging 

station at Luray (figs. 1, 8), and about 4 mi upstream from the 

Luray Dam Reservoir. Transects 3 and 4 at the Kauffman Mill 

reach represent some of the last free-flowing water before the 

reservoir alters flow in the channel. The most downstream 

reach is Thunderbird Farms, located about 6 mi upstream from 

Front Royal (figs. 1, 9). Thunderbird Farms is mostly natural 

pool habitat (table 6) with a smooth bedrock bottom, which 

does not provide much cover or diversity in habitat for fish. 

Standard PHABSIM data-collection procedures (Bovee, 

1997) were used to collect channel and floodplain topographic 

information for each reach. One or two elevation benchmarks 

were set and surveyed using a GPS with submeter accuracy 

per reach as control points for a total station survey of transect 

markers and channel features. The GPS benchmarks for each 

reach allowed transect bed elevations and water-surface levels 

to be tied to a common elevation datum. Horizontal and 

vertical surveys of benchmarks, transect-marker elevations, 

and channel bed elevations were conducted with a surveying 

level or a total station, with an accuracy of 0.02 to 0.1 ft within 

a given reach. The surveys provided the distance between 

transects, water-surface slope, bed-elevation profiles, stage 

of zero-flow elevation, water depths, and benchmark locations and elevations. The primary benchmarks used when the 

transects were initially surveyed were maintained during the 

project and served as elevation control for WSL measurements 

during hydraulic data collection. 

EXPLANATION

Transect location

 and number

Model-grid habitat

 sampling cell

3

 Model grid cells are 10 feet across.

XS

1

2

3

Digital Orthophoto source: Virginia Geographic Information Network, 2007, VBMP Orthophotography—2006/2007 

(VA State Plane North), 1:2,400 scale (1”= 200’), available online at http://www.isp.virginia.gov/vbmporthophotography.shtml

Figure 7. Transect locations at Riverbend Pool on the South Fork of the Shenandoah River, Virginia. 

Locations of study sites are shown on figure 1. 

Jen Krstolic marking GPS waypoints on South Fork 

Shenandoah River
Mesohabitat Summary of the South Fork Shenandoah River 15

EXPLANATION

Transect location

 and number

Model-grid habitat

 sampling cell

4

 Model grid cells are 10 feet across.

0 50 ft 

XS

0

1

2

3

4

XS

1

2

3

4

4

 Model grid cells are 10 feet across.

EXPLANATION

Transect location

 and number

Model-grid habitat

 sampling cell

Digital Orthophoto source: Virginia Geographic Information Network, 2007, VBMP Orthophotography—2006/2007 

(VA State Plane North), 1:2,400 scale (1”= 200’), available online at http://www.isp.virginia.gov/vbmporthophotography.shtml

Figure 9. Transect locations at Thunderbird Farms on the South Fork of the Shenandoah River, Virginia. 

Locations of study sites are shown on figure 1

Digital Orthophoto source: Virginia Geographic Information Network, 2007, VBMP Orthophotography—2006/2007 

(VA State Plane North), 1:2,400 scale (1”= 200’), available online at http://www.isp.virginia.gov/vbmporthophotography.shtml

Figure 8. Transect locations at Kauffman Mill on the South Fork of the Shenandoah River, Virginia. 

Locations of study sites are shown on figure 1. 
16 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

Hydraulic Data-Collection Methods

Measurements of hydraulic data were made over a range 

of streamflows representative of the summer flow conditions at 

four study sites to provide calibration data for the RHABSIM 

physical habitat model. Measurements included WSL (or stage), 

water depth, water velocity, discharge, and substrate and cover 

characteristics relevant to fish habitat needs on the South Fork. 

Hydraulic measurements were made six or more times at all 

transects between May 2007 and August 2009 to represent 

summer low-flow, medium-flow, and high-flow conditions 

to develop a stage-discharge rating for each site. RHABSIM 

requires one discharge to represent the flow for a given reach 

for each day, even though the discharge measured at each 

transect may vary slightly. The modeling software refers to the 

representative reach discharge as the “best-estimate” discharge. 

Five best-estimate discharges that represent the widest flow 

range and most accurate stage-discharge relation for each 

site were selected and used as calibration discharges for the 

modeling phase of this research (table 7). 

For each day of data collection, WSLs were measured 

with a surveying level to the nearest 0.001 ft at each transect before and after data collection. WSLs and observed 

discharges were checked each day in the field at Lynnwood 

and compared to the gage discharge and stage to ensure 

agreement of measured datasets. WSL differences between 

measured and gage stage for transect 5 ranged between 0.01 

and 0.16 ft with a median value of 0.045 ft. Because of the 

difficulty of surveying stage in the field at Lynnwood, these 

numbers are likely the widest differences of all monitored 

study reaches. This check was helpful in assessing accuracy in 

WSL measurements at Lynnwood, but was not an option at the 

other study reaches where no gages were present. 

A tagline was stretched across each transect, and water 

depths and velocities were recorded every 10 ft at the specified verticals for the reach. Depth and mean column velocity 

were measured with one of the following: a Flow Tracker 

(SonTek YSI, Incorporated, 2006; calibrated to the nearest 

0.01 ft/s), with a StreamPro (RD Instruments) Acoustic 

Doppler Current Profiler (ADCP) in “section-by-section” 

mode (calibrated to the nearest 0.01 ft/s); or a Price AA and 

Pygmy meter (calibrated to the nearest 0.01 ft/s). Depth 

and velocity measurements followed USGS procedures for 

discharge measurements (Buchanan and Somers, 1969), 

except that the spacing between velocity readings, or verticals, 

was constant. For all except the lowest discharges at a few 

sites, the ADCP “moving boat method” (Mueller and Wagner, 

2009; Turnipseed and Sauer, 2010) was used to determine 

the best-estimate discharge in one transect per reach, because 

this is an approved method for calculating discharge (Mueller 

and Wagner, 2009). For the lowest discharges, Price AA or 

FlowTracker depth and velocity measurements from one 

transect per reach were used to calculate the best-estimate 

reach discharge. All transects were measured on the same day 

for a reach, but reaches were usually measured on different 

days during the same week. 

Fish-Community Sampling and 

Microhabitat Observations

Fish habitat-use observations were conducted at USGS 

hydraulic data-collection reaches and at two additional 

sites (fig. 1) during June, July, August, and September of 

2008. Flows during this timeframe were between the 10th

and 75th percentile for each month, but most frequently at 

the 25th percentile flow (Ramey, 2009). Each study reach 

contained three to five transects, with each transect at a 

discrete mesohabitat type. Specific transects to be sampled 

for fish habitat use at each study reach were stratified by 

mesohabitat type and randomly selected in proportion to 

the relative abundance of each habitat type. A single study 

reach could contain more than one transect sampled. In total, 

eight transects at six study reaches were sampled using three 

sampling methodologies.

Data-Collection Methods

Fish habitat suitability criteria were developed with data 

from all three collection methods. The first method utilized 

a roving observer following sampling lanes as described 

in Bovee (1986) and Persinger (2003); the second method 

utilized a stationary observer in randomly selected sampling 

cells as described in Ramey (2009). Both methods used direct 

underwater observation (Goldstein, 1978; Thurow, 1994) 

to observe microhabitat use by fishes. Underwater visibility 

was considered adequate for sampling if an observer with 

mask and snorkel could see at least 3 ft to each side and from 

the water surface to the substrate. The third data-collection 

method was prepositioned electroshocking of areas sampled 

by the stationary observer (Ramey 2009). 

Table 7. Best-estimate discharges used in the River Habitat 

Simulation model (RHABSIM) for each hydraulic data-collection reach on the South Fork Shenandoah River, Virginia.

Reach name

Calibration discharge, in cubic feet 

per second

1 2 3 4 5

Lynnwood 181 221 725 780 1,132

Riverbend Pool 271 380 554 962 1,187

Kauffman Mill 358 631 890 1,540 2,087

Thunderbird Farms 327 349 1,123 2,014 2,064
Fish-Community Sampling and Microhabitat Observations 17

Observed fish were identified, counted, and size class 

recorded. In addition to the specified observation period 

while snorkeling, 10 rocks were flipped in each sampling 

unit (lanes or cells) to look for cryptic taxa including 

margined madtoms. A conscious effort was made not to 

count the same fish repeatedly, not to count fish that were 

obviously attracted to the observer, nor to count fish that 

were observed in microhabitats outside of the boundaries of 

sample cells or lanes.

Depth (measured to the nearest 0.1 ft) and mean column 

velocity (measured to the nearest 0.1 ft/s at 0.6 x depth) 

were measured with a FlowTracker (Sontek) on a graduated 

top-setting wading rod or with a StreamPro (RD Instruments) 

ADCP at the locations of the rover observer’s fish markers and 

at the center of each of the 10 sample cells observed by the 

stationary observer. 

Numerical codes for dominant and subdominant substrate 

size (table 8) based on a modified Wentworth scale (Bovee, 

1986), cover (table 9), and embeddedness (see table 13 of 

Krstolic and others, 2006), were used so that substrate characterizations could be summarized in a single “channel index” 

(Krstolic and others, 2006). Where two or more cover types 

were present at a particular location, cover was characterized 

as “complex” in the channel index. Embeddedness was 

Table 8. Substrate codes used to classify 

the dominant and subdominant substrate 

within fish study reaches and hydraulic 

data-collection reaches on the South Fork 

Shenandoah River, Virginia.

[Code modified from Krstolic and others, 2006. 

NA, not applicable; >, greater than; <, less than]

Code Substrate 

category

Size, 

millimeters

1 Organic debris NA

2 Silt NA

3 Sand < 2

4 Fine gravel 3 to 8

5 Small gravel 9 to 22

6 Large gravel 23 to 64

7 Small cobble 65 to 128

8 Large cobble 129 to 256

9 Small boulder 257 to 512

10 Large boulder > 512

11 Bedrock NA

Electroshocking Crew—Clay and Julie 

Ramey, and Charlie Simmons (Virginia 

Department of Environmental Quality)
18 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

estimated visually as one of four categories: 0 to less than 

25 percent, 25 to less than 50 percent, 50 to less than 

75 percent, and 75 to 100 percent. Although channel index 

was evaluated for composite HSC rankings for each fish, what 

appeared to be the most important component of the channel 

index was the dominant substrate type. Therefore, for fish that 

had HSC defined, dominant substrate suitability was evaluated 

in the habitat suitability simulations, instead of channel index. 

With few exceptions, fishes were identified to species. 

Spotfin and satinfin shiners (Cyprinella spiloptera and 

C. analostana) were indistinguishable from one another while 

swimming and were recorded as Cyprinella spp. Young-of-year 

of all species were assumed to be using specific habitat types; 

therefore, young-of-year were recorded as such without finerscale distinction. Visually apparent life stages were recorded 

for juvenile, sub-adult, and adult when possible, and generally 

only for the species that attained the largest adult sizes. 

Development and Testing of Fish Habitat 

Suitability Criteria

Habitat suitability criteria were evaluated only for taxa 

sampled on 30 or more events (following Maki-Petays and 

others, 2002), with the exception of adult smallmouth bass, 

which had an event sample size of only 18. Although 48 

species or life stages were observed at least once during the 

study (Ramey, 2009), HSC were only evaluated for 9 species 

and life stages that had adequate sample size: sub-adult smallmouth bass and adult smallmouth bass (Micropterus dolomieu), 

juvenile redbreast sunfish, sub-adult redbreast sunfish, and 

adult redbreast sunfish (Lepomis auritus), satinfin or spotfin 

shiner (Cyprinella spp.), margined madtom (Noturus insignis), 

river chub (Nocomis micropogon), and young-of-year. 

The habitat-use observations for depth, velocity, or 

channel index (substrate and cover) for each species were 

evaluated to determine ranges of optimal, marginal, suitable, 

and unsuitable habitat criteria (tables 10–12). Optimal habitats 

were defined as those within 25 percent of the median value, 

or the central 50 percent of the observed habitat parameter. 

Marginal habitats were represented by 15 percent greater than 

or less than the range of the central 50 percent of the observed 

habitat parameter. Habitats used by the central 80 percent of 

each taxa and life stage sampled (optimal + marginal) were 

Table 9. Codes used to classify the cover 

found around fish observations and within 

hydraulic data-collection reaches on the 

South Fork Shenandoah River, Virginia.

[Code modified from Krstolic and others, 2006]

Code Description

1 No cover

2 Overhead limbs

3 Small woody debris

4 Large woody debris

5 Interstitial space

6 Boulder

7 Tilted bedrock

8 Submerged aquatic vegetation

9 Complex

Smallmouth bass (Micropterus dolomieu)

Table 10. Suitable range of depths for fish in the South Fork 

Shenandoah River, Virginia. 

[Marginal, depths used by fish representing the central 80 percent of data; 

Optimal, depths used by fish representing the central 50 percent of data; 

n, number of samples]

Taxa/life stage n 

(events)

Suitable range of depths, 

in feet

Lower Upper

Marginal Optimal Marginal

Sub-adult smallmouth bass 61 1.3 1.7 2.9 5.5

Smallmouth bass 145 2.1 2.8 5.5 6.2

Juvenile redbreast sunfish 31 0.8 1.1 1.9 2.4

Sub-adult redbreast sunfish 31 1.3 1.8 3.6 4.4

Redbreast sunfish 30 0.9 1.6 3.5 4.5

Cyprinella spp. 61 0.9 1.1 1.8 2.4

Margined madtom 30 0.7 0.9 1.6 2.9

River chub 54 0.1 1.1 2.1 2.9

Young-of-year 37 0.9 1.1 2.0 3.7

1Used individual sample size for smallmouth bass (event n = 18).
Fish-Community Sampling and Microhabitat Observations 19

Table 11. Suitable range of velocity for fish in the South Fork 

Shenandoah River, Virginia. 

[Marginal, velocity used by fish representing the central 80 percent of data; 

Optimal, velocity used by fish representing the central 50 percent of data; 

n, number of samples]

Taxa/life stage n 

(events)

Suitable range of velocity, 

in feet per second

Lower Upper

Marginal Optimal Marginal

Sub-adult smallmouth bass 61 0.13 0.26 1.21 1.77

Smallmouth bass 145 0.05 0.17 0.69 1.17

Juvenile redbreast sunfish 31 0.01 0.16 0.90 1.28

Sub-adult redbreast sunfish 31 0.06 0.13 0.64 1.77

Redbreast sunfish 30 0.05 0.20 0.72 1.17

Cyprinella spp. 61 0.16 0.44 1.28 2.20

Margined madtom 30 0.08 0.64 1.43 2.96

River chub 54 0.14 0.58 1.64 2.35

Young-of-year 37 0.05 0.18 1.08 1.76

1Used individual sample size for smallmouth bass (event n = 18).

Table 12. Suitable range of channel index for fish in the South 

Fork Shenandoah River, Virginia. 

[Marginal, channel index used by fish representing the central 80 percent of 

data; Optimal, channel index used by fish representing the central 50 percent 

of data; n, number of samples]

Taxa/life stage n 

(events)

Suitable range of channel index

Lower Upper

Marginal Optimal Marginal

Sub-adult smallmouth bass 61 36.84 67.92 119.92 611.94

Smallmouth bass 145 37.94 76.92 210.94 211.94

Juvenile redbreast sunfish 31 29.94 64.93 87.51 611.91

Sub-adult redbreast sunfish 31 28.94 67.51 117.91 210.94

Redbreast sunfish 30 33.94 63.53 117.91 711.91

Cyprinella spp. 61 56.91 67.52 117.91 611.91

Margined madtom 30 67.83 76.63 117.91 611.91

River chub 54 67.51 76.91 118.91 611.92

Young-of-year 37 28.94 37.94 116.91 611.91

1Used individual sample size for smallmouth bass (event n = 18).

(α = 0.05). The mean depth or velocity values for occupied 

habitats for sub-adult redbreast sunfish and adult redbreast 

sunfish were not significantly different, so only the sub-adult 

redbreast sunfish were evaluated. The final HSC developed 

and tested in this study include sub-adult smallmouth bass and 

adult smallmouth bass, juvenile redbreast sunfish, sub-adult 

redbreast sunfish, and adult redbreast sunfish, Cyprinella spp., 

margined madtom, and river chub (app. 1).

Canoeing Suitability Criteria

Canoeing suitability criteria are based on the same depth 

and velocity ranges used for the North Fork and Shenandoah 

River mainstem studies (for criteria, see Zappia and Hayes, 

1998; or Krstolic and others, 2006). The canoeing suitability 

criteria were created based on water depths considered to 

be adequate for passage (enough water to avoid stranding) 

across riffle areas. A minimum depth of 1 ft was considered 

marginal and given a suitability index value of 0.5. Depths 

greater than 2.5 ft were considered optimal and given a 

suitability index value of 1.0, indicating greater suitability for 

canoeing. Because excessive water velocities are considered 

hazardous for all but the most skilled paddlers, the velocity 

suitability index had an opposite pattern to depth. Velocity 

within the range of 0.5 to 2.5 ft/s were considered optimal 

and given an index value of 1.0, whereas velocities faster than 

2.5 ft/s were considered marginal, and had decreasing index 

values. Velocities above 5 ft/s were considered unsuitable for 

canoeing. All substrates were considered suitable. 

considered suitable, whereas unsuitable habitats were those 

with habitat parameters with ranges outside of those used by 

the central 80 percent of each taxa and life stage (following 

Persinger, 2003; Ramey, 2009). 

To test whether the fish observations represented a 

preferential use of habitats, Student’s T–tests were conducted 

for occupied and unoccupied habitats for each species with 

an adequate sample size. Student’s T–test of occupied and 

unoccupied habitats showed that there were significant differences in depths where fish were present than depths where 

fish were not present for all species except young-of-year 

and sub-adult smallmouth bass (α = 0.05) (fig. 10). Student’s 

T–test of occupied and unoccupied habitats also showed that 

there were significant differences in velocities where fish were 

present than velocities where fish were not present for all 

species except young-of-year and sub-adult smallmouth bass 

and Cyprinella spp. (α = 0.05) (fig. 10). These basic statistics 

provide insight to the fact that the fish were located in habitats 

preferentially and that their habitat selection was not likely to 

be random. Since young-of-year represent multiple species, it 

makes sense that they would not preferentially use a particular 

depth or velocity range, although the assumption had been 

that small fish would seek shelter in similar areas. Sub-adult 

smallmouth bass are generalist species, usually utilizing any 

available habitat to obtain their prey. In this study, however, 

separate data for sub-adult smallmouth bass and adult smallmouth bass were used in the habitat modeling, because when 

occupied habitat characteristics were evaluated between 

lifestages, both velocity and depth were significantly different 
20 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

RHABSIM Modeling to Determine 

Fish Habitat Availability and 

Recreation Conditions

The program RHABSIM 3.0 for DOS and Windows 

(Thomas R. Payne and Associates, 1998) was used for 

calibration and simulation of flow and habitat. The RHABSIM 

modeling process includes two calibration phases and three 

simulation phases. Following the same methods used in 

Krstolic and others (2006), the WSL and velocity calibrations were completed first, followed by WSL, velocity, and 

habitat simulations. Calibration and simulation phases were 

performed separately for each reach, then in the upper reach 

habitat simulation results for two sites were combined.

Model Calibration

The RHABSIM model calibration uses the datasets 

collected during the hydraulic data-collection part of the 

study. Calibration involves inputting topographic information for each study reach, WSLs, velocities, and discharge 

data for each hydraulic data-collection reach. The transect 

data are used to calculate stage-discharge ratings to enable 

simulation of depths and velocities for flows not measured 

during hydraulic data collection. Data input requirements for 

the South Fork were identical to the North Fork (Krstolic and 

others, 2006, p. 24). Because model calibration techniques 

were the same as those used for the North Fork project, 

only deviations of reach-specific details are presented in the 

sections that follow. 

Water-Surface-Level Calibration

WSL models were selected for transects within each reach 

based on reach habitat conditions and hydraulic properties 

(table 13). As in Krstolic and others (2006) each model used 

the measured (observed) WSL, one discharge measurement 

per reach as the “best-estimate calibration discharge,” and 

user-defined n-values or beta-values (hydraulic roughness 

parameters, as specified by the particular model). The hydraulic 

0

0.20

0.40

0.60

0.80

1.00

1.20

1.40

AVERAGE VELOCITY, IN FEET PER SECOND

A

B

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

AVERAGE DEPTH, IN FEET

FISH SPECIES

Sub-adult redbreast

sunfish

Redbreast sunfish

Juvenile redbreast

sunfish

Smallmouth bass

Margined madtom

River chub

Cyprinella spp.

Sub-adult smallmouth

bass

Young-of-year

Occupied (not significant)

Occupied

Unoccupied

Occupied (not significant)

Occupied

Unoccupied

EXPLANATION

EXPLANATION

Figure 10. A, Average depth and 

B, average velocity data input to 

Student’s T-test of occupied and 

unoccupied habitats for each species 

of the South Fork Shenandoah River, 

Virginia. Blue or green bars represent 

statistically significant differences 

between occupied and unoccupied 

habitats used by fish.
RHABSIM Modeling to Determine Fish Habitat Availability and Recreation Conditions 21

roughness parameters were used to adjust simulated conditions 

so that they accurately represent instream conditions by 

producing a simulated WSL that matched the measured value. 

The model calibration was an iterative process, with new 

n-values or beta-values selected to minimize the difference 

between the observed and simulated WSL (Waddle, 2001). 

For sites with pool habitat where WSL did not vary greatly 

over the length of the reach (Thunderbird Farms and Riverbend Pool), the difference between observed and simulated 

WSL was less than or equal to 0.02 ft. For the two sites with 

more heterogeneous habitat and riffles in the central part of 

the reach (Kauffman Mill and Lynnwood), the difference 

between observed and simulated WSL ranged from 0.01 to 

0.14 ft, depending on the simulation discharge and habitat 

at a given transect. 

To assess accuracy of flows higher than calibration 

discharges, field data for WSL were compared to simulated 

WSL for selected high discharges. The Lynnwood simulation 

discharge of 3,840 ft3/s was surveyed for WSL only, as the 

conditions were too high to collect data in the river. The simulated WSL were fairly consistent for transects 1 and 2 with the 

surveyed WSL having a difference in range of 0 and 0.25 ft 

of the simulated WSL. The simulated WSL for transects 3, 

4, and 5 were much lower than those surveyed in the field, 

with a WSL difference from 1.8 to 2 ft less than the simulated 

WSL. This indicates that the model is underpredicting depths 

for higher flows. The error in simulated WSL increases with 

discharge for calibration flows, and continues to increase as 

flows increase. The discharge of 3,840 ft3/s was simulated to 

check the accuracy of the simulations, considering that this 

flow represents a 95th annual percentile flow (table 2), and is 

not likely to be important in the evaluation of flow thresholds 

for summer low-flow conditions. 

For Kauffman Mill, a similar exercise was conducted 

with a dataset that was not used for calibration to compare 

simulated WSL to field surveyed WSL. The 2,087 ft3/s 

field-surveyed WSL data were compared to simulated WSL, 

resulting in a difference of 0.003 to 0.39 ft. This discharge 

was much closer in magnitude to the highest calibration 

discharge, which likely accounts for the smaller differences 

in simulated and measured WSLs. 

Table 13. Water-surface-level calibration methods and velocity calibration discharges used in the River Habitat Simulation model for 

each hydraulic data-collection reach and transect on the South Fork Shenandoah River, Virginia.

[WSL, water-surface level; ft3/s, cubic feet per second; WSP, water-surface-profile method; MANSQ, Manning stage-discharge method; NA, not applicable. 

Transects are numbered in upstream to downstream order, beginning with number 0 or 1]

Reach Transect Habitat 

type

WSL 

calibration 

method1

WSL

calibration 

discharge, 

ft3/s

Beta,

(MANSQ)

Manning’s n 

(WSP)

Velocity 

calibration 

discharge, 

ft3/s

Calculated 

discharge 

from 

simulated 

velocities, 

ft3/s

Ratio 

difference: 

velocity 

adjustment 

factor

Percent 

difference

Lynnwood 1 Run, bedrock WSP 780 NA 0.056 780 802 0.97 2.7

2 Riffle, particle WSP 780 NA 0.25 780 748 1.04 -4.3

3 Run, bedrock MANSQ 780 0.3993 NA 1,132 1,235 0.92 8.3

4 Run, bedrock MANSQ 780 0.5187 NA 780 812 0.96 3.9

5 Run, bedrock MANSQ 780 0.5136 NA 780 839 0.93 7.0

Riverbend Pool 1 Pool, bedrock WSP 1,187 NA 0.0431 1,187 1,309 0.91 9.3

2 Pool, bedrock WSP 1,187 NA 0.13 1,187 797 1.49 -48.9

3 Pool, bedrock WSP 1,187 NA 0.13 1,187 1,238 0.96 4.1

Kauffman Mill 0 Glide WSP 1,540 NA 0.0596 890 924 0.96 3.7

1 Rifffle, particle WSP 1,540 NA 0.035 358 928 0.39 61.4

2 Run, particle MANSQ 631 0.48 NA 890 877 1.01 -1.5

3 Run, bedrock MANSQ 631 0.62 NA 2,087 2,815 0.74 25.9

4 Run, bedrock MANSQ 631 0.64 NA 2,087 1,175 1.78 -77.6

Thunderbird 

Farms

1 Pool, bedrock WSP 404 NA 0.05 404 410 0.99 1.5

2 Pool, bedrock WSP 404 NA 0.0503 1,123 1,238 0.91 9.3

3 Glide WSP 404 NA 0.1209 1,123 1,220 0.92 8.0

4 Riffle, bedrock WSP 404 NA 0.0275 1,123 1,213 0.93 7.4

1From Thomas R. Payne and Associates, 1998.
22 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

Although the literature would allow for extrapolation 

higher than the highest discharge measured, the accuracy 

of simulations declines greatly outside the bounds of 

the measured flows. For this investigation, the model 

application is related to low flows and conditions less than 

the 25th percentile flow. Should these models be used for 

other purposes, the underprediction of depths as flows 

increase outside the calibration discharge range should 

be considered. 

When the WSL model calibration parameters were 

finalized and the WSLs had been simulated for the calibration discharges, the model was used to simulate WSLs for 

a set of 30 discharges representative of the flows within 

the 5th percentile to greater than the 95th percentile annual 

flow for each management section of the river (table 14). 

Simulation flows were spaced by about 100 ft3/s to create 

regular intervals for simulation. Because the drainage 

areas associated with each of the hydraulic data-collection 

reach were within 1 percent of the drainage areas for the 

nearest gaging station, the model simulation flows were not 

corrected for drainage area differences between the hydraulic 

data-collection reach and the streamflow-gaging station.

Velocity Calibration

Velocity calibration procedures were very similar 

to those used with the North Fork models (Krstolic 

and others, 2006). The 1‐velocity calibration method 

(Thomas R. Payne and Associates, 1998) was selected for 

all reaches and transects. For each transect, the velocity 

dataset that produced the least difference between the 

best-estimate discharge and model-calculated discharge 

was selected as the primary dataset to calibrate the 

velocity model (table 13). Cell by cell roughness values 

were adjusted to ensure a good match between the simulated velocity and measured velocity calibration dataset. 

Measured velocities for each model cell in the velocity 

calibration dataset were used as a template and adjusted 

based on the predicted depths from WSL simulations 

for higher or lower discharges. The model-calculated 

discharge for each transect was compared to best-estimate 

calibration discharge for the reach to obtain velocity 

adjustment factors (VAF) for each calibration discharge. 

After VAF were determined for calibration discharges, 

simple regressions were completed to incrementally 

increase the VAF for each simulation discharge. VAF were 

used to scale the velocity calibration dataset to simulate 

velocities for the 30 simulation flows. VAF were largest 

for transects with discharges that differ greatly from the 

best-estimate discharge, such as riffle transects.

Table 14. Range of discharges used in the River Habitat 

Simulation model for simulations of water-surface levels, 

velocities, and habitats on the South Fork Shenandoah River, 

Virginia.

[Blue font values represent calibration flows]

Discharge, in cubic feet per second

Lynnwood1 Riverbend Pool2 Kauffman

Mill3

Thunderbird 

Farms4

72 108 124 130

122 181 174 179

181 238 224 228

221 271 262 290

265 380 358 327

337 424 380 349

360 481 470 404

460 554 520 450

544 600 570 557

604 690 631 599

725 747 681 698

780 804 731 831

880 861 769 897

980 918 890 948

1,050 962 996 1,123

1,132 1,032 1,140 1,213

1,200 1,187 1,290 1,312

1,300 1,327 1,440 1,412

1,400 1,469 1,540 1,490

1,500 1,584 1,700 1,760

1,600 1,698 1,800 1,908

1,900 1,790 2,089 2,064

2,100 1,869 2,390 2,360

2,400 2,040 2,500 2,444

2,600 2,212 2,770 2,742

2,800 2,554 3,070 3,160

3,000 2,725 3,250 3,734

3,400 2,897 3,550 4,230

3,840 3,000 3,850 4,650

4,200 3,068 No data 5,160

1The simulation discharges for Lynnwood represent a range between 

0.40 times the lowest calibration flow and 3.7 times greater than the highest 

calibration flow. 

2The simulation discharges for Riverbend Pool represent a range between 

0.40 times the lowest calibration flow and 2.6 times greater than the highest 

calibration flow. 

3The simulation discharges for Kauffman Mill represent a range between 

0.35 times the lowest calibration flow and 2.5 times greater than the highest 

calibration flow. 

4The simulation discharges for Thunderbird Farms represent a range 

between 0.40 times the lowest calibration flow and 2.5 times greater than 

the highest calibration flow. 
RHABSIM Modeling to Determine Fish Habitat Availability and Recreation Conditions 23

Habitat Simulation and Development of 

Weighted Usable-Habitat Area Curves

Model cells input to RHABSIM for each study reach 

were centered on the verticals where depths and velocities 

were collected at 10‐ft intervals, and substrate or cover 

measurements were made for 5 ft on either side of the center 

point. The length of model cells was varied based on habitat 

represented by the transect and the percentage of the reach 

upstream or downstream of the transect that contained the 

same habitat type. Transect weighting factors (app. 2) were 

used to determine model cell lengths, which are used to 

determine cell areas for available habitat. Figures 6–9 show 

variation in cell lengths among study sites. 

The WSL and velocity simulations and the fish and 

canoeing HSC were input to the Habitat Simulation Model 

(HABSIM) of RHABSIM (Thomas R. Payne and Associates, 

1998). HABSIM uses the HSC suitable ranges for water 

depths, water velocities, and dominant substrate to assign 

individual suitability ranks (on a scale from 0.0 to 1.0) for 

depth, velocity, and substrate in each model cell. Multiplicative aggregation (Waddle, 2001) was used to calculate the 

composite suitability rank for each cell, which also produced a 

composite score between 0.0 and 1.0. The area of all suitable 

habitat cells within a reach was summed for a total weighted 

usable-habitat area (WUA). The process was repeated for each 

species or lifestage of fish and canoeing over 30 simulation 

flows, and a functional relation between habitat and discharge 

was defined and expressed in the form of WUA curves.

For recreation, particularly canoeing, adequate water 

depth to avoid scraping bottom is a major consideration. 

Therefore, habitat simulations for canoeing had additional 

restrictions placed on depth to include only cells that are 1‐ft 

deep or greater. No restrictions on velocity suitability, other 

than the HSC suitability ranks, were implemented. 

As was done in the North Fork study, the South Fork 

Shenandoah River basin study reaches were placed in upper, 

middle, and lower sections that are associated with the 

streamflow-gaging stations (Lynnwood, Luray, and Front 

Royal) near the downstream end of each section. Unlike the 

North Fork where the discharge doubles between streamflowgaging stations, the median annual streamflow only increases 

by 224 ft3/s (145 Mgal/d) between Lynnwood and Luray, and 

increases by 120 ft3/s (78 Mgal/d) between Luray and Front 

Royal. The largest amount of water withdrawals occur in 

the Lynnwood section, which has the least amount of water 

availability, so two hydraulic study reaches were placed in this 

section. The other two downstream sections each have one 

study reach. Transects in each reach were weighted (app. 2) 

to represent the total percentage of habitat available in that 

section of the river based on data from table 4.

Habitat-Discharge Relations for StreamflowGaging Stations on the South Fork

Lynnwood Ecological Habitat

For this investigation, the Lynnwood streamflow-gaging 

station and hydraulic data-collection reaches in this section 

represent discharge in the reach between the confluence of the 

North, Middle, and South Rivers up to the Shenandoah Dam. 

Lynnwood and Riverbend Pool study reaches were located 

in this section. The Lynnwood reach contains run and riffle 

habitats, whereas the Riverbend Pool contains only pool habitat. 

A RHABSIM model was calibrated for WSL and velocities 

collected from field data and evaluated against fish HSC 

(tables 10–12; app. 2) for the seven species or lifestages of fish 

discussed earlier in this document. The amount of available 

usable habitat, weighted for the percentage of each type of 

habitat present in this section of the river to generate WUA, was 

calculated for a range of streamflows from 72 to 4,200 ft3/s for 

the Lynnwood reach and 108 to 3,068 ft3/s for Riverbend Pool 

(table 14). The WUA for each reach and the combined upper 

section of river is shown in tables 15–17, and figs. 11–13. 

To illustrate the amount of WUA commonly expected for 

each fish during the low-flow period, the JAS normal range 

of flows (265 to 544 ft3/s), the 10th percentile flow (203 ft3/s) 

for JAS flows, the 5th percentile flow (178 ft3/s) for JAS flows, 

and the annual 7Q10 statistic (151.0 ft3/s) were plotted over 

each set of WUA curves (figs. 11–13). Riverbend Pool and 

Lynnwood individual reach model result figures are shown 

(figs. 12, 13) to illustrate how different habitat types (a pool, 

a riffle or run, respectively) produced varying amounts of 

WUA for each species. However, this discussion involves 

the combined curves (Riverbend Pool + Lynnwood) for the 

upper section so that pools, riffles, and runs are all represented. 

The normal range of flows for JAS included the maximum 

WUA for sub-adult redbreast sunfish and sub-adult smallmouth 

bass, whereas WUA curves increased for Cyprinella spp., river 

chub, margined madtom, and juvenile redbreast sunfish as flows 

decreased within the normal range of flows for JAS. Lower 

flows provided habitat for smaller, riffle-dwelling fishes, as 

indicated by the WUA curve peaks which occurred at 181 ft3/s 

for river chub and at 221 ft3/s for Cyprinella spp. and margined 

madtom. The maximum WUA for juvenile redbreast sunfish 

occurred at flows lower than the 5th percentile flow. By the time 

flows reached the 5th percentile for JAS flow, habitat availability 

was declining for all species except juvenile redbreast sunfish. 

Adult smallmouth bass habitat availability was lower than 

that for any other species in the combined reaches (fig. 11) 

modeled for the upper section of the river, but was relatively 

abundant in the Riverbend Pool reach (fig. 13). The WUA in 

the Riverbend Pool reach illustrates adult smallmouth bass 

dependence on deeper pool habitats, but also shows the upper 

boundary of depths that are ”too deep” based on observations 

(or lack of fish presence). The rest of the species studied did not 

exhibit a preference for deep-water habitats found at Riverbend 

Pool and would likely be found only at the edges.
24 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

Table 15. Weighted usable-habitat area, in square feet per 1,000 feet of stream, for the upper section, including the Lynnwood study 

reach and the Riverbend Pool study reach on the South Fork Shenandoah River, Virginia. 

[Mgal/d, million gallons per day; ft3/s, cubic feet per second; ND, no data simulated for this flow]

Simulated 

discharge, 

Mgal/d

Simulated 

discharge, 

ft3/s

Ecological habitat Recreational

habitat

Fish species

Canoeing Sub-adult 

smallmouth 

bass

Adult 

smallmouth 

bass

Juvenile 

redbreast 

sunfish

Sub-adult 

redbreast 

sunfish

Cyprinella

spp.

Margined 

madtom River chub

46.5 72 17,427 6,837 50,640 33,645 23,682 22,613 50,936 38,107

78.8 122 32,815 11,507 55,812 57,955 39,982 30,712 68,261 74,287

117.0 181 38,663 19,562 50,261 54,144 42,725 40,795 95,909 100,214

142.8 221 40,852 18,718 47,006 52,869 43,374 42,614 93,934 114,191

217.8 337 45,906 19,722 26,672 62,867 31,985 38,639 89,171 155,042

232.7 360 45,640 21,350 24,061 64,635 31,002 39,382 84,704 168,022

297.3 460 33,355 25,210 15,551 59,452 23,284 32,178 73,694 195,838

351.6 544 29,385 25,586 11,788 57,373 18,129 30,376 62,465 206,560

390.3 604 26,542 24,470 11,387 51,576 13,829 26,254 51,513 218,549

468.5 725 19,037 24,414 10,138 42,112 12,269 21,859 34,829 227,526

504.1 780 13,806 23,102 10,259 33,872 9,828 20,644 29,469 234,868

568.7 880 11,365 23,463 8,973 28,631 7,718 17,162 26,014 234,014

633.3 980 9,469 26,332 8,387 26,610 7,073 16,275 21,347 233,035

678.6 1050 9,836 24,833 9,489 25,447 6,721 15,431 18,407 236,647

731.6 1132 11,318 26,089 10,321 27,507 5,637 14,643 16,277 237,306

775.5 1200 11,931 24,606 9,670 29,071 4,727 13,263 12,077 235,581

840.2 1300 8,853 22,034 7,535 25,780 3,377 11,824 11,056 228,542

904.8 1400 8,027 21,093 7,036 24,754 3,718 11,200 9,945 220,004

969.4 1500 7,434 20,181 7,048 23,849 3,738 8,744 10,093 213,028

1,034.0 1600 8,202 19,280 6,861 23,264 4,354 6,460 9,863 207,297

1,227.9 1900 6,875 21,323 5,376 19,999 2,900 3,845 8,111 201,796

1,357.2 2100 6,693 19,947 5,191 17,470 2,784 3,930 7,816 196,307

1,551.1 2400 7,066 20,686 5,301 16,716 3,027 4,319 6,255 185,476

1,680.3 2600 7,607 21,790 4,518 18,488 1,903 3,540 6,263 176,471

1,809.6 2800 3,815 3,304 2,076 7,203 1,903 3,173 5,313 ND

1,938.8 3000 3,834 3,304 2,203 6,945 1,168 3,556 4,224 ND

2,197.3 3400 2,904 1,124 1,457 6,872 671 3,637 3,289 ND

2,481.7 3840 3,144 2,828 3,975 8,233 3,006 6,099 6,402 ND

2,714.3 4200 3,634 3,211 3,669 8,331 3,008 5,529 5,733 ND
RHABSIM Modeling to Determine Fish Habitat Availability and Recreation Conditions 25

Table 16. Weighted usable-habitat area, in square feet per 1,000 feet of stream, for the Riverbend Pool study reach on the South 

Fork Shenandoah River, Virginia.

[Mgal/d, million gallons per day; ft3/s, cubic feet per second]

Simulated 

discharge, 

Mgal/d

Simulated 

discharge, 

ft3/s

Ecological habitat Recreational

habitat

Fish species

Canoeing Sub-adult 

smallmouth 

bass

Adult 

smallmouth 

bass

Juvenile 

redbreast 

sunfish

Sub-adult 

redbreast 

sunfish

Cyprinella

spp.1

Margined 

madtom1 River chub1

117 181 1,254 8,751 4,255 9,190 0 0 0 1,164

154 238 1,376 10,762 3,806 8,999 0 0 0 9,191

175 271 2,180 11,658 4,953 10,258 0 0 0 16,722

246 380 3,751 12,543 4,644 14,836 0 0 0 44,255

274 424 3,848 15,265 4,629 15,845 0 0 0 55,215

311 481 4,591 18,052 4,014 17,525 0 0 0 65,464

358 554 4,340 21,956 3,645 16,692 0 0 0 75,392

388 600 4,239 22,334 3,528 16,483 0 0 0 82,004

446 690 5,198 21,580 3,583 15,957 0 0 0 93,036

483 747 5,175 22,410 2,961 15,826 0 0 0 98,389

520 804 4,591 22,969 2,661 17,116 0 0 0 104,146

557 861 4,776 21,662 2,551 15,911 0 195 0 109,755

594 918 4,694 21,657 2,888 16,112 0 402 0 114,347

622 962 4,587 21,873 3,357 16,209 0 414 0 117,736

667 1,032 4,177 24,364 2,147 14,974 0 414 0 121,450

768 1,187 4,171 22,477 2,210 13,807 0 0 0 129,461

858 1,327 4,249 22,477 2,199 15,349 0 0 0 136,430

950 1,469 5,828 21,651 1,959 17,631 0 0 0 140,973

1,024 1,584 4,970 19,144 1,843 16,454 0 0 0 143,599

1,098 1,698 4,378 18,203 1,328 15,041 0 194 0 145,980

1,158 1,790 3,423 17,292 1,174 14,552 0 371 0 147,785

1,209 1,869 3,315 17,666 1,183 14,295 0 420 0 149,447

1,319 2,040 3,315 19,955 1,296 12,534 0 420 0 152,250

1,431 2,212 3,240 18,950 1,693 10,729 0 285 0 154,008

1,652 2,554 3,315 17,473 2,173 9,843 0 0 0 152,099

1,762 2,725 3,384 17,473 2,845 10,247 0 0 0 147,072

1,874 2,897 4,202 18,486 2,372 11,660 0 0 0 141,527

1,940 3,000 4,397 18,320 2,262 11,398 0 0 0 138,950

1,984 3,068 4,619 18,320 2,267 10,127 0 0 0 137,305

1Shallow-water species habitat was limited by water depths that were too deep even at the lowest flows simulated. 
26 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

Table 17. Weighted usable-habitat area, in square feet per 1,000 feet of stream, for the Lynwood study reach on the South Fork 

Shenandoah River, Virginia.

[Mgal/d, million gallons per day; ft3/s, cubic feet per second]

Simulated 

discharge, 

Mgal/d

Simulated 

discharge, 

ft3/s

Ecological habitat Recreational

habitat

Fish species

Canoeing Sub-adult 

smallmouth 

bass

Adult 

smallmouth 

bass

Juvenile 

redbreast 

sunfish

Sub-adult 

redbreast 

sunfish

Cyprinella

spp.

Margined 

madtom River chub

79 122 31,562 2,756 51,557 48,765 39,982 30,712 68,261 73,124

117 181 37,288 8,800 46,455 45,144 42,725 40,795 95,909 91,023

143 221 38,672 7,060 42,053 42,611 43,374 42,614 93,934 97,469

171 265 42,102 8,896 34,913 45,687 39,481 41,203 93,079 103,297

218 337 42,156 7,179 22,027 48,031 31,985 38,639 89,171 110,787

233 360 41,792 6,085 19,433 48,790 31,002 39,382 84,704 112,807

297 460 29,014 3,253 11,906 42,760 23,284 32,178 73,694 120,446

352 544 25,146 3,252 8,259 40,889 18,129 30,376 62,465 124,556

390 604 21,344 2,890 7,803 35,618 13,829 26,254 51,513 125,513

469 725 14,446 1,445 7,476 24,996 12,269 21,859 34,829 123,380

504 780 9,111 1,445 7,371 17,760 9,828 20,241 29,469 120,521

569 880 6,778 1,590 5,616 12,422 7,718 16,748 26,014 116,279

633 980 5,292 1,968 6,240 11,636 7,073 15,862 21,347 111,585

679 1050 5,665 2,356 7,279 11,640 6,721 15,431 18,407 107,186

732 1132 7,069 3,612 8,122 12,157 5,637 14,643 16,277 100,876

776 1200 6,103 2,955 7,711 11,440 4,727 13,263 12,077 94,609

840 1300 3,883 2,890 5,693 9,326 3,377 11,824 11,056 84,942

905 1400 3,649 2,890 5,709 9,713 3,718 11,006 9,945 74,024

969 1500 4,011 2,890 5,874 9,297 3,738 8,372 10,093 65,244

1,034 1600 4,888 1,613 5,678 8,969 4,354 6,040 9,863 57,850

1,228 1900 3,635 2,373 3,683 9,270 2,900 3,560 8,111 47,788

1,357 2100 3,378 2,474 3,018 7,627 2,784 3,930 7,816 44,208

1,551 2400 3,682 3,213 2,456 6,469 3,027 4,319 6,255 38,404

1,680 2600 3,406 3,304 2,146 6,828 1,903 3,540 6,263 34,944

1,810 2800 3,815 3,304 2,076 7,203 1,903 3,173 5,313 32,174

1,939 3000 3,834 3,304 2,203 6,945 1,168 3,556 4,224 30,199

2,197 3400 2,904 1,124 1,457 6,872 671 3,637 3,289 27,080

2,482 3840 3,144 2,828 3,975 8,233 3,006 6,099 6,402 23,447

2,714 4200 3,634 3,211 3,669 8,331 3,008 5,529 5,733 20,093
RHABSIM Modeling to Determine Fish Habitat Availability and Recreation Conditions 27

Figure 11. Weighted usable-habitat area of ecological and recreational habitat combined for the Lynnwood and Riverbend 

Pool study reaches on the South Fork of the Shenandoah River, Virginia. Flow percentiles are based on data from the 

Lynnwood streamflow-gaging station (01628500) from 1930 to 2008 for streamflows during the months of July, August, and 

September. These statistics represent the expected range of flows, based on the historic streamflow record.

Sub-adult smallmouth bass

Adult smallmouth bass

Juvenile redbreast sunfish

Sub-adult redbreast sunfish

Cyprinella spp.

Margined madtom

River chub

Canoeing

 50 151 178 203 265 360 544 1,000 4,000 

Mgal/d 32 98 115 131 171 233 352 646 2,585 7Q10

ft3/s

0

50,000

100,000

150,000

200,000

250,000

Weighted usable habitat area, in square feet per 1,000 feet

Discharge in cubic feet per second (ft3/s) or million gallons per day (Mgal/d)

Game fish habitat

Nongame fish habitat

Recreational (canoe) habitat

July–August–September

 flow percentiles

>75%

25%–75%

10%–<25%

5%–<10%

<5%

EXPLANATION

Ecological (fish) habitat

Bedrock riffle, South Fork Shenandoah River
28 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

Figure 12. Weighted usable-habitat area of ecological and recreational habitat at the Lynnwood study reach on the South 

Fork of the Shenandoah River, Virginia. Flow percentiles are based on data from the Lynnwood streamflow-gaging station 

(01628500) from 1930 to 2008 for streamflows during the months of July, August, and September. These statistics represent the 

expected range of flows, based on the historic streamflow record. 

Sub-adult smallmouth bass

Adult smallmouth bass

Juvenile redbreast sunfish

Sub-adult redbreast sunfish

Cyprinella spp.

Margined madtom

River chub

Canoeing

 50 151 178 203 265 360 544 1,000 4,000 

Mgal/d 32 98 115 131 171 233 352 646 2,585 7Q10

ft3/s

0

25,000

50,000

75,000

100,000

125,000

150,000

Weighted usable habitat area, in square feet per 1,000 feet

Discharge in cubic feet per second (ft3/s) or million gallons per day (Mgal/d)

Game fish habitat

Nongame fish habitat

Recreational (canoe) habitat

July–August–September

 flow percentiles

>75%

25%–75%

10%–<25%

5%–<10%

<5%

EXPLANATION

Ecological (fish) habitat

Downstream view, South Fork Shenandoahh River near Lynnwood
RHABSIM Modeling to Determine Fish Habitat Availability and Recreation Conditions 29

Figure 13. Weighted usable-habitat area of ecological and recreational habitat at the Riverbend Pool study reach on 

the South Fork of the Shenandoah River, Virginia. Simulation results had no available habitat for cyprinella spp., margined 

madtom, or river chub at this site. Flow percentiles are based on data from the Lynnwood streamflow-gaging station 

(01628500) from 1930 to 2008 for streamflows during the months of July, August, and September. These statistics represent 

the expected range of flows, based on the historic streamflow record.

Sub-adult smallmouth bass

Adult smallmouth bass

Juvenile redbreast sunfish

Sub-adult redbreast sunfish

Cyprinella spp.

Margined madtom

River chub

Canoeing

Game fish habitat

Nongame fish habitat

Recreational (canoe) habitat

July–August–September

 flow percentiles

>75%

25%–75%

10%–<25%

5%–<10%

<5%

EXPLANATION

Ecological (fish) habitat

 50 151 178 203 265 360 544 1,000 4,000 

Mgal/d 32 98 115 131 171 233 352 646 2,585 

ft3

/s

0

25,000

50,000

75,000

100,000

125,000

150,000

Weighted usable habitat area, in square feet per 1,000 feet

Discharge in cubic feet per second (ft3/s) or million gallons per day (Mgal/d)

7Q10

Dennis Adams making an ADCP measurement along the 

South Fork Shenandoah River
30 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

Canoeing through the Lynnwood Section

Although two study reaches are included in the 

Lynnwood section, only the Lynnwood study reach has 

riffle habitat. Riverbend Pool is all pool habitat and should 

not present any difficulties for canoeing, although the slow 

velocity range (0.2 to 0.5 ft/s for flows less than 1,050 ft3/s) 

suggest that the paddler may have to do more paddling in 

this section. The Lynnwood study reach contains one riffle at 

transect 2, which was used to identify flows that would limit 

depths suitable for canoe passage. Discharge equal to 360 ft3/s 

represents the lowest discharge that will produce an average 

depth of 1 ft for the transect, a maximum depth of 2.37 ft, and 

an average velocity of 2.1 ft/s. Although the average depth is 

equal to 1 ft, almost half of the study cells within the transect 

will not meet this requirement (fig. 14A). These conditions 

are what can be expected for the 50th percentile flow for JAS 

(360 ft3/s, table 2). A discharge of 460 ft3/s ensures that the 

majority of the cells across the channel width are greater than 

or equal to 1‐ft deep. For a discharge of 460 ft3/s, the average 

depth is 1.2 ft, the maximum depth is 2.6 ft, and the average 

velocity is 2.4 ft/s (fig. 14B). The Lynnwood study reach has 

bedrock ledges that run parallel to flow, which channelize 

water and create focused areas of faster velocity. Where this 

form of bedrock occurs along transect 3, the average velocities reach 4.1 ft/s at 2,400 ft3/s and 5.1 ft/s at 3,400 ft3/s. 

The Lynnwood study reach WUA curves for canoeing (fig. 12) 

show that the maximum canoeing habitat occurs with a 

discharge of 604 ft3/s, and a decline in canoeing habitat occurs 

at flows greater than 604 ft3/s. 

Luray Ecological Habitat

For this investigation, the Luray streamflow-gaging 

station and Kauffman Mill hydraulic data-collection reach 

represent discharge and habitat for the stretch of river from 

the Shenandoah Dam to the Luray Dam. As mentioned 

previously, Kauffman Mill contains glide, run, and riffle 

habitats. A RHABSIM model was calibrated for water-surface 

levels and velocities that were collected from field data and 

evaluated against HSC (tables 10–12; app. 1) for the seven 

species or lifestages of fish discussed earlier in this document. The amount of available usable habitat, weighted for 

the percentage of each type of habitat present in the river, 

WUA, was calculated for a range of streamflows from 124 to 

3,850 ft3/s (table 14). The WUA for Kauffman Mill and the 

middle section of the river is shown in table 18 and figure 15. 

To illustrate the amount of WUA commonly expected 

for each fish during the low-flow period, the JAS normal 

range of flows (380 to 769 ft3/s), the 10th percentile flow 

(310 ft3/s) for JAS flows, the 5th percentile flow (262 ft3/s) 

for JAS flows, and the annual 7Q10 statistic (225.0 ft3/s) 

were plotted over each set of WUA curves (fig. 15). The 

normal range of flows for JAS includes the maximum 

WUA for all fish except juvenile and sub-adult redbreast 

sunfish. For these species the maximum WUA occurs close 

to the 25th percentile flow, and thereafter, habitat availability 

declines as flows decline. Maximum WUA for sub-adult and 

juvenile redbreast sunfish occurs with flows lower than the 

5th percentile. Adult smallmouth bass habitat availability 

is less than all the other species in this reach; however, the 

overall WUA for smallmouth bass is almost double that of 

the Riverbend Pool reach. The low WUA in the Kauffman 

Mill reach illustrates smallmouth bass dependence on deep 

pool habitats (on the order of 5 to 6 ft deep), but not as deep 

as the majority of Riverbend Pool (15 ft). River chub and 

margined madtom habitat availability is maximized at this 

site, followed closely by sub-adult smallmouth bass. At 

Kauffman Mill, habitat availability seems to be maintained 

with flows as low as the 10th percentile flow (310 ft3/s) 

(fig. 15). When flows reach the annual 7Q10 flow statistic, 

habitat declines are observed for all species. 

Canoeing through the Luray Section 

An examination of each transect, as well as the WUA 

reach-wide results for Kauffman Mill, help to put the canoeing 

WUA results into context. Kauffman Mill has a high-gradient 

riffle at transect 1 that is a limiting feature for paddlers during 

low-flow conditions. This riffle is considered to be representative of larger riffles in the Luray section. Flows equal 

to 520 ft3/s provide an average depth of 1.3 ft, a maximum 

depth of 2.17 ft, and an average velocity of 1.3 ft/s for the 

riffle at transect 1. Flows equal to 262 ft3/s (the 5th percentile 

flow for JAS) provide adequate depths at only a few sections 

along transect 1. To ensure that transect 1 is passable along 

the majority of the channel width, paddlers may want to select 

days when discharge is equal to or greater than 520 ft3/s. For a 

flow of 520 ft3/s, the combined depth and velocity suitability 

criteria are met for 75 percent of the reach area (fig. 16A). 

The other transects at Kauffman Mill are run or glide habitats 

and should not create difficulties for paddlers. These modeling 

results are close to the minimum discharge suggested by local 

outfitter organizations who offer river trips between Luray and 

Front Royal. One company suggests that 404 ft3/s at the Luray 

gage is very low, and that ideal paddling conditions are 830 

to 3,070 ft3/s (Downriver Canoe Co., 2011). The RHABSIM 

modeling results suggest that the maximum canoeing habitat 

occurs at 890 ft3/s (table 18, figs. 15–16B). Model results and 

outfitters agree that flows near 800 ft3/s represent suitable 

canoeing conditions for the Luray section of the South Fork.

Front Royal Ecological Habitat

For this investigation the Front Royal streamflow-gaging 

station and Thunderbird Farms hydraulic data-collection reach 

measurements are used to represent discharge in the reach 

between Luray to the gage at Front Royal. As mentioned 

previously, Thunderbird Farms contains pool, glide, and 

riffle habitats. A RHABSIM model was calibrated for watersurface levels and velocities collected from field data and 

evaluated against HSC (tables 10–12; app. 1) for the seven 

species or lifestages of fish discussed earlier in this document. The amount of available usable habitat, weighted for 
RHABSIM Modeling to Determine Fish Habitat Availability and Recreation Conditions 31

-50 3 57 110 163 217 270

-50 3 57 110 163 217 270

-85 13 58 130 202 273 345

1.00

0.83

0.67

0.50

0.33

0.17

0

1.00

0.83

0.67

0.50

0.33

0.17

0

1.00

0.83

0.67

0.50

0.33

0.17

0

Depth suitability rank Depth suitability rank Velocity suitability rank

4.50

3.75

3.00

2.25

1.50

0.75

0

4.50

3.75

3.00

2.25

1.50

0.75

0

Velocity, in feet per second Velocity, in feet per second Velocity, in feet per second

6.700

5.588

4.471

3.353

2.235

1.118

0

Distance from left bank to right bank, in feet

Distance from left bank to right bank, in feet

Distance from left bank to right bank, in feet

A

B

C

EXPLANATION

Model cell simulated velocity

River channel elevation profile

Water-surface elevation

Model cell suitability

1,012.3 ft.

1,012.3 ft.

1,030.4 ft.

1,031.9 ft.

1,025.7 ft.

1,011.6 ft.

1,030.4 ft.

1,016.7 ft.

1,016.7 ft.

Figure 14. Canoeing suitability for selected transects at the Lynnwood study reach on the South Fork of the 

Shenandoah River, Virginia. A, depth suitability and simulated velocity for riffle habitat at transect 2 associated 

with the simulated discharge equal to 360 cubic feet per second (ft3/s); B, depth suitability and simulated velocity 

for riffle habitat at transect 2 associated with the simulated discharge equal to 460 ft3/s; and C, velocity suitability 

and simulated velocity for run habitat at transect 3 associated with simulated discharge equal to 2,400 ft3/s.
32 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

Table 18. Weighted usable-habitat area, in square feet per 1,000 feet of stream, for the Kauffman Mill study reach on the South Fork 

Shenandoah River, Virginia.

[Mgal/d, million gallons per day; ft3/s, cubic feet per second]

Simulated 

discharge, 

Mgal/d

Simulated 

discharge, 

ft3/s

Ecological habitat Recreational

habitat

Fish species

Sub-adult Canoeing

smallmouth 

bass

Adult smallmouth bass

Juvenile 

redbreast 

sunfish

Sub-adult 

redbreast 

sunfish

Cyprinella

spp.

Margined 

madtom River chub

112 174 75,455 40,168 86,230 106,770 64,890 66,794 90,985 111,873

145 224 83,507 45,167 84,438 105,670 71,211 81,842 113,923 136,749

169 262 88,481 42,553 80,758 96,426 73,329 95,841 127,891 148,590

231 358 93,541 44,804 65,469 85,830 70,273 103,739 136,132 166,599

246 380 96,930 41,731 63,593 83,898 72,728 105,912 138,240 169,396

304 470 100,278 39,626 43,813 88,399 63,416 101,780 135,104 178,946

336 520 93,898 35,945 41,420 88,559 62,671 98,254 128,742 183,288

368 570 85,738 31,954 34,826 84,569 56,807 90,372 123,595 186,812

408 631 76,707 24,606 31,257 83,220 54,141 84,460 116,461 190,616

472 731 66,784 20,345 21,063 79,085 45,554 74,339 100,662 194,917

497 769 62,950 19,270 19,078 78,035 41,395 70,564 96,683 196,199

575 890 48,991 14,366 16,042 63,520 32,813 60,379 76,787 197,442

644 996 40,341 13,421 12,744 51,244 23,442 52,125 65,357 195,648

737 1,140 32,099 13,436 10,199 41,465 19,405 42,260 50,591 190,758

834 1,290 25,446 9,982 9,999 34,463 16,629 34,222 38,840 183,541

931 1,440 22,542 9,958 8,827 28,561 13,184 29,446 31,080 171,779

995 1,540 18,562 8,128 8,882 22,126 10,759 27,234 25,442 159,930

1,099 1,700 16,726 6,570 9,258 19,610 5,870 23,974 22,572 141,571

1,163 1,800 15,627 6,858 9,656 18,592 5,072 21,949 21,817 131,761

1,350 2,089 14,261 7,157 10,321 18,736 5,808 19,474 15,835 108,796

1,480 2,290 12,531 8,045 10,135 18,209 4,437 17,456 13,246 97,229

1,616 2,500 12,401 8,047 8,645 17,579 4,431 14,763 11,159 85,166

1,790 2,770 12,098 9,984 8,730 17,111 6,305 12,319 9,825 71,029

1,984 3,070 11,516 10,389 9,922 16,095 6,106 10,955 8,958 60,247

2,100 3,250 11,758 8,269 10,082 15,944 6,357 9,486 8,857 55,231

2,294 3,550 11,477 8,163 9,755 17,043 6,084 9,335 9,615 49,516

2,488 3,850 11,799 7,245 7,554 16,739 3,803 8,533 10,501 44,572
RHABSIM Modeling to Determine Fish Habitat Availability and Recreation Conditions 33

the percentage of each type of habitat present in the river, 

WUA, was calculated for a range of streamflows from 130 

to 5,160 ft3/s (table 14). The WUA for Thunderbird Farms 

and the Front Royal sections of river is shown in table 19 and 

figure 17. 

To illustrate the amount of WUA commonly expected 

for each fish during the low-flow period, the JAS normal 

range of flows (420 to 830 ft3/s), the 10th percentile flow 

(330 ft3/s) for JAS flows, the 5th percentile flow (290 ft3/s) for 

JAS flows, and the annual 7Q10 statistic (247.4 ft3/s) were 

plotted over each set of WUA curves (fig. 17). The normal 

range of flows for JAS includes the maximum WUA for 

all fish except juvenile and sub-adult redbreast sunfish. 

The maximum WUA for sub-adult redbreast sunfish occurs 

at slightly less than the 25th percentile flow; the juvenile 

redbreast sunfish WUA continues to increase with decreasing 

discharge to the lowest flow simulated. For pool and rundwelling fish like smallmouth bass and redbreast sunfish, 

habitat availability does not vary much from the normal range 

of flows for the low-flow period, down to the 5th percentile 

flow (290 ft3/s). Below the 5th percentile, habitat sharply 

decreases. For riffle-dwelling fish, the normal range of flows 

maintains habitat availability, but below the 25th percentile 

flow (420 ft3/s), decreases and then remains steady (fig. 17). 

Habitat declines as flows decline for all species, except 

juvenile redbreast sunfish, as flows decrease below the 

annual 7Q10. 

Canoeing through the Front Royal Section

Canoeing through the Front Royal section takes the 

paddler through the widest channels on the South Fork 

and extensive sections of bedrock riffle like that present in 

transect 4 of the Thunderbird Farms study reach, or throughout 

Andy Guest State Park (Krstolic and Hayes, 2010). These 

bedrock riffles typically run perpendicular to the flow and are 

usually areas of shallow-water depth. To represent adequate 

canoeing suitability throughout this section, transect 4 was 

examined in detail. RHABSIM modeling results show that the 

minimum flow needed to maintain an average depth greater 

than or equal to 1 ft is 557 ft3/s (50 th percentile JAS flow), 

Mgal/d

ft3

/s 100 225 262 310 380 491 769 1,000 4,000

64.6 145.5 169 200 246 317 497 646.5 2,560

Usable habitat area, in square feet per 1,000 feet

Discharge in cubic feet per second (ft3/s) or million gallons per day (Mgal/d)

0

50,000

100,000

150,000

200,000

7Q10

Sub-adult smallmouth bass

Adult smallmouth bass

Juvenile redbreast sunfish

Sub-adult redbreast sunfish

Cyprinella spp.

Margined madtom

River chub

Canoeing

Game fish habitat

Nongame fish habitat

Recreational (canoe) habitat

July–August–September

 flow percentiles

>75%

25%–75%

10%–<25%

5%–<10%

<5%

EXPLANATION

Ecological (fish) habitat

Figure 15. Weighted usable-habitat area of ecological and recreational habitat at the Kauffman Mill study reach on 

the South Fork of the Shenandoah River, Virginia. Flow percentiles are based on data from the Luray streamflow gaging 

station (01629500) from 1979 to 2008 for streamflows during the months of July, August, and September. These statistics 

represent the expected range of flows, based on the historic streamflow record. 
34 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

A

B

0

1

2

3

4

612

375

301

103

0

Distance upstream, in feet

612

375

301

103

0

Distance upstream, in feet

Transect number

0

1

2

3

4

Transect number

Distance from right bank, in feet

–75 4 83 163 242 321 400

Distance from right bank, in feet

–75 4 83 163 242 321 400

100% suitable Unsuitable

EXPLANATION

= 1.0 >0.8 >0.6 >0.4 >0.2 >0 0

Suitability indices greater than 0.5 represent depths equal to or greater

than 1.0 foot. Green, yellow, orange, and red are suitable. 

Figure 16. Model grid of canoeing depth suitability for the Kauffman 

Mill study reach on the South Fork of the Shenandoah River, Virginia. 

A, 520 cubic feet per second; and B, 890 cubic feet per second.
RHABSIM Modeling to Determine Fish Habitat Availability and Recreation Conditions 35

Table 19. Weighted usable-habitat area, in square feet per 1,000 feet of stream, for the Thunderbird Farms study reach on the South Fork 

Shenandoah River, Virginia.

[Mgal/d, million gallons per day; ft3/s, cubic feet per second]

Simulated 

discharge, 

Mgal/d

Simulated 

discharge, 

ft3/s

Ecological habitat Recreational

habitat

Fish species

Canoeing Sub-adult 

smallmouth 

bass

Adult smallmouth bass

Juvenile 

redbreast 

sunfish

Sub-adult 

redbreast 

sunfish

Cyprinella

spp.

Margined 

madtom River chub

116 179 67,074 78,247 63,581 137,844 27,384 27,685 67,815 19,202

147 228 82,051 103,090 59,826 141,063 26,112 27,685 67,986 45,060

187 290 92,037 111,543 54,082 147,624 27,062 30,810 65,698 86,310

211 327 98,800 117,935 49,318 151,019 32,640 37,827 66,442 111,613

226 349 99,237 119,360 47,753 152,973 32,661 39,944 67,192 127,612

261 404 100,757 122,957 44,642 150,962 32,292 40,421 74,491 161,079

291 450 101,158 134,361 40,101 149,028 30,668 44,275 88,490 184,831

360 557 99,539 136,537 29,804 124,686 40,276 55,827 84,775 229,761

387 599 97,207 135,187 30,700 121,897 39,168 54,529 85,485 241,543

451 698 93,053 124,338 25,531 110,154 37,524 51,199 83,390 263,244

537 831 87,990 111,994 15,061 92,280 29,857 47,717 76,312 282,499

580 897 86,327 100,986 13,342 90,323 26,522 43,117 72,172 288,851

613 948 83,194 93,889 12,503 88,796 25,340 41,003 66,933 292,700

726 1,123 64,779 71,440 8,511 82,795 19,271 35,302 52,389 298,234

784 1,213 62,225 68,782 6,828 81,543 18,704 32,114 49,498 304,162

848 1,312 57,198 63,374 6,134 77,455 17,051 27,285 44,262 306,880

913 1,412 52,724 57,478 5,325 73,306 14,381 19,319 37,669 308,701

963 1,490 50,044 52,516 5,286 71,042 13,550 15,797 33,229 309,529

1,137 1,760 36,712 26,498 3,940 59,067 7,141 11,931 19,750 311,273

1,233 1,908 31,617 18,105 3,589 52,789 6,293 11,465 15,046 311,780

1,334 2,064 25,643 11,101 2,162 37,367 4,754 10,722 14,210 314,180

1,525 2,360 19,096 7,389 2,193 31,425 2,634 8,927 9,435 308,636

1,579 2,444 18,458 7,389 2,549 30,427 2,634 8,629 8,407 307,575

1,772 2,742 14,299 6,649 2,749 21,088 2,055 5,831 4,490 302,946

2,042 3,160 6,294 3,790 1,569 7,881 1,284 4,549 2,441 292,816

2,413 3,734 3,811 1,810 1,221 6,595 771 1,928 2,055 268,418

2,734 4,230 2,037 1,818 1,290 3,335 386 1,547 781 247,716

3,005 4,650 1,826 1,409 1,224 3,864 0 971 964 226,476

3,335 5,160 1,411 2,066 839 3,499 0 386 193 199,321
36 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

with an average depth of 1.1 ft, a maximum depth of 1.9 ft, 

and an average velocity of 1.3 ft/s (fig. 18A). A discharge 

of 698 ft3/s provides an average depth of 1.3 ft, a maximum 

depth of 1.98 ft, and an average velocity of 1.5 ft/s, whereas 

a discharge of 831 ft3/s (75th percentile JAS flow) provides 

an average depth of 1.4 ft, a maximum depth of 2.1 ft, and 

an average velocity of 1.6 ft/s (fig. 18B). Considering the 

fact that this kind of habitat is typically a series of bedrock 

ledges of varying depth, a paddler may have more success 

with flows that have average depths equal to or greater than 

1.4 ft. Outfitter suggestions presented in the Luray section 

also apply to the Front Royal section (Downriver Canoe Co., 

2011). Modeling results show that the maximum suitable 

canoeing habitat occurs at 2,064 ft3/s (fig. 17, table 19), and 

as flows increase velocity suitability decreases. Average and 

maximum velocities associated with a discharge of 3,160 ft3/s 

at transect 4 are 3.1 and 4.8 ft/s respectively. When depth and 

velocity are considered, a suitable range for canoeing may be 

from 831 up to 3,160 ft3/s. 

Mgal/d

ft3

/s 100 247 290 330 420 557 831 5,000

64.6 160 187 213 271 537

1,000

646 3,231

10,000

360 6,463

7Q10

Usable habitat area, in square feet per 1,000 feet

Discharge in cubic feet per second (ft3/s) or million gallons per day (Mgal/d)

0

50,000

100,000

150,000

200,000

250,000

300,000

350,000

Sub-adult smallmouth bass

Adult smallmouth bass

Juvenile redbreast sunfish

Sub-adult redbreast sunfish

Cyprinella spp.

Margined madtom

River chub

Canoeing

Game fish habitat

Nongame fish habitat

Recreational (canoe) habitat

July–August–September

 flow percentiles

>75%

25%–75%

10%–<25%

5%–<10%

<5%

EXPLANATION

Ecological (fish) habitat

Figure 17. Weighted usable-habitat area of ecological and recreational habitat at Thunderbird Farms in the lower reach 

on the South Fork Shenandoah River, Virginia. Flow percentiles are based on data from the Front Royal streamflow-gaging 

station (01631000) from 1931 to 2008 for streamflows during the months of July, August, and September. These statistics 

represent the expected range of flows, based on the historic streamflow record.

Don Hayes measures flow in a riffle at 

Kauffman Mill during August 2009
RHABSIM Modeling to Determine Fish Habitat Availability and Recreation Conditions 37

–40 50 140 230 320 410 500

–40 50 140 230 320 410 500

1.00

0.83

0.67

0.50

0.33

0.17

0

1.00

0.83

0.67

0.50

0.33

0.17

0

Combined suitability rank Combined suitability rank

1.905

1.588

1.270

0.953

0.635

0.318

0

Velocity, in feet per second

2.320

1.933

1.547

1.160

0.773

0.387

0

Velocity, in feet per second

Distance from left bank to right bank, in feet

Distance from left bank to right bank, in feet

A

B

EXPLANATION

Model cell simulated velocity

River channel elevation profile

Model cell suitability

492.7 ft.

503.3 ft.

499.7 ft.

492.7 ft.

503.3 ft.

499.7 ft.

Figure 18. Canoeing suitability for the bedrock riffle at transect 4 at the Thunderbird Farms 

study reach on the South Fork Shenandoah River, Virginia. Combined depth and velocity 

suitability ranks (bars) and simulated velocity (green line) at a discharge of A, 557 cubic feet 

per second; and B, 831 cubic feet per second.
38 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

Habitat Time-Series Scenario Analysis for 

Low-Flow Periods

Time-series plots of the habitat availability can be 

calculated from the discharge-habitat relations and applied 

to historic daily flows for streamflow-gaging stations on 

the South Fork to provide a picture of habitat availability 

over all flows measured. Drought years were examined as 

examples of times when habitat was potentially limited so that 

species and habitats affected by drought could be identified. 

This assessment sought to describe the flow conditions that 

were potentially stressful for each species or lifestage and to 

describe normal or ideal conditions related to habitat availability. The results and descriptions are interpretations of the 

weighted usable-habitat area curves, the time-series plots, and 

the historic flow record. The scenarios presented in this report 

are for informational purposes only, and any final selection of 

flow thresholds and desired habitat availability during drought 

or low-flow periods would belong to the purview of managers, 

planners, and policymakers in the Shenandoah Valley. 

Historic droughts represent rare conditions that require 

planning and preparation to ensure enough water availability 

for citizens and to maintain healthy rivers. Five historic 

summer drought low-flow periods (1932, 1966, 1977, 1999, 

and 2002) were examined for each study site when data were 

available. The most extreme drought on record occurred in 

2002, so that year was used for time-series examples for each 

river section. Other time periods were presented when they 

helped define thresholds or the effects of water withdrawal 

scenarios. For withdrawal scenarios, river conditions had to 

reach extreme lows (less than 10th or 5th percentile JAS flow) 

before withdrawal increases or water-conservation scenarios 

showed noticeable effect on habitat availability. 

During historic droughts, adequate flows for canoeing 

rarely occurred. For instance, along the middle section of 

the South Fork, flows less than the 25th percentile flow for 

JAS do not provide adequate depth for passage through riffle 

habitats. For the time-series scenarios presented in this report, 

flow conditions were between the 25th and 5th percentile 

JAS flows, but generally less than the 10th percentile flow 

for JAS. Although recreation is certainly a consideration for 

water-resources management, when flows are far below the 

25th percentile and decreasing, canoe paddling is unlikely to be 

successful, even with reduced water withdrawals. 

Upper Section Time-Series Scenarios

The upper section of the South Fork flows from the 

confluence of the North, Middle, and South Rivers to the 

Shenandoah Dam Reservoir. The annual median flow for 

this section of river is 604 ft3/s (390.4 Mgal/d, table 2). 

The normal range of flows for JAS is 265 to 544 ft3/s (171.3 to 

351.6 Mgal/d, table 2). Habitat time-series plots for game and 

nongame fish during 2002 and 1977 are presented in figures 

19 and 20. 

Streamflows were below normal from June 1, 2002, to 

October 1, 2002; the discharge ranged from 448 to 84 ft3/s 

with all but 12 days less than 265 ft3/s (the 25th percentile 

flow for JAS). This time period represents an extreme drought 

scenario for the Lynnwood streamflow-gaging station. 

Maximum WUA values occur just below the 25th percentile 

JAS flow for nongame fish, and within the normal range 

of flows for all game fish except juvenile redbreast sunfish 

(fig. 11, table 15). During 2002, habitat availability for all 

species, except juvenile redbreast sunfish, decreased between 

40 to 70 percent for a given species when flows decreased 

from 200 to 84 ft3/s. Beginning June 15, 2002, when streamflow at the Lynnwood gage was 205 ft3/s (about 10th percentile 

flow; fig. 19), decreases in habitat occurred for all species 

except juvenile redbreast sunfish. Figure 19 shows expected 

normal range of habitat availability, or JAS weighted usablehabitat area percentiles, as well as the time-series results 

for 2002. The JAS weighted usable-habitat area percentiles 

represent a historic summary (such as exceedance values for 

streamflows) of habitat throughout JAS. The 50th percentile 

or median habitat condition is shown as well as the normal 

habitat range (25th to 75th percentile of habitat). These habitat 

percentiles illustrate that habitat availability during 2002 

generally was less than the median for JAS and during the 

driest times less than the 25th habitat percentile. The habitat 

decrease was most pronounced in early July and midAugust as flows decreased below 100 ft3/s. In the upper reach 

Cyprinella spp. and sub-adult redbreast sunfish habitat availability were the least affected by low-flow conditions. This is 

the only section of the South Fork that showed even marginal 

declines in sub-adult redbreast sunfish habitat. Flows less than 

100 ft3/s are much less than the 5th percentile of flow for JAS 

(178 ft3/s, table 2) and are typically rare; however, time-series 

data from 2002 show that extreme low-flow conditions corresponded to below-normal habitat availability for both game 

and nongame fish for extended periods of time in the upper 

section of the river. 

Streamflows during 1977 remained near the 10th percentile 

JAS flow for 62 days, and below the 5th percentile for 26 days, 

which represent a less intense drought than 2002. Habitat 

time-series comparisons between 2002 and 1977 (figs. 19, 

20), showed that the 10th percentile flow (203 ft3/s) was an 

indicator of some habitat limitation, but when flows remained 

at the 10th percentile, as they did in 1977, habitat area did not 

decrease greatly. JAS weighted usable-habitat area percentiles 

overlaid on figure 20 confirm that habitat conditions were close 

to the 50th percentile for most days in 1977. Habitat time-series 

simulations only resulted in a decrease for adult smallmouth 

bass during 1977 (fig. 20). 
RHABSIM Modeling to Determine Fish Habitat Availability and Recreation Conditions 39

June July August

2002

September October

2

3

4

5

x 104

1

2

3

x 104

2

3

4

5

x 104

4

6

8

10

x 104

102

103

Discharge, in cubic

feet per second Weighted usable habitat area per 1,000 feet of stream

 100

 1,000

 200

 50

Habitat time series upper section South Fork Shenandoah River, Historic Flows, 2002

Discharge at the Lynnwood Streamflow-Gaging Station, 2002

Sub-adult smallmouth bass

Adult smallmouth bass

Margined madtom

River chub

Game fish habitat

Nongame fish habitat

EXPLANATION

A

B

C

D

E

Normal range of habitat for

 July–August–September

75% habitat percentile

50% habitat percentile

25% habitat percentile

Discharge

Figure 19. Habitat time series for the upper section of the South Fork Shenandoah River near Lynnwood, Virginia, 

during 2002. A and B, game fish weighted usable-habitat area; C and D, nongame fish weighted usable-habitat area; 

and E, daily mean discharge for the river near Lynnwood (01628500).
40 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

June July August September October

2

3

4

5

x 104

1

2

3

x 104

2

3

4

5

x 104

4

6

8

10

x 104

102

103

Discharge, in cubic

feet per second Weighted usable habitat area per 1,000 feet of stream

 100

 1,000

 200

 50

Habitat time series upper section South Fork Shenandoah River, Historic Flows, 1977

Discharge at the Lynnwood Streamflow-Gaging Station, 1977

1977

Sub-adult smallmouth bass

Adult smallmouth bass

Margined madtom

River chub

Game fish habitat

Nongame fish habitat

EXPLANATION

A

B

C

D

E

Normal range of habitat for

 July–August–September

75% habitat percentile

50% habitat percentile

25% habitat percentile

Discharge

Figure 20. Habitat time series for the upper section of the South Fork Shenandoah River near Lynnwood, Virginia, 

during 1977. A and B, game fish weighted usable-habitat area; C and D, nongame fish weighted usable-habitat area; 

and E, daily mean discharge for the river near Lynnwood (01628500).
RHABSIM Modeling to Determine Fish Habitat Availability and Recreation Conditions 41

Middle Section Time-Series Scenarios

The middle section of the South Fork flows from the 

Shenandoah Dam to the Luray Dam. The annual median flow 

for this section of river is 828 ft3/s (535.2 Mgal/d, table 2). 

The normal range of flows for JAS is 380 to 769 ft3/s (245.6 

to 497 Mgal/d, table 2). Combined habitat time-series plots 

for selected species or lifestages during 2002 and 1999 are 

presented (figs. 21, 22). 

Streamflows were below normal from June 1, 2002, to 

October 1, 2002; the discharge ranged from 163 to 624 ft3/s 

with all but 12 days less than the 25th percentile flow for JAS 

(380 ft3/s, fig. 21). During 2002, habitat availability decreased 

for all species except juvenile redbreast sunfish and sub-adult 

redbreast sunfish. Because adult smallmouth bass, margined 

madtom, and river chub maximum WUA all occurred in 

association with the 25th percentile flow (fig. 15), a decrease 

in streamflow below 380 ft3/s corresponded with a decrease 

in habitat availability. The greatest decrease in habitat availability for smallmouth bass, sub-adult smallmouth bass, 

margined madtom, and river chub occurred between July 1, 

2002, and July 26, 2002, when flows decreased below the 

5th percentile of flow for JAS (262 ft3/s, fig. 21). In the middle 

reach, the habitat discharge relation shows about a 10‐percent 

reduction in available habitat for each species between the 

25th percentile and the 5th percentile flow. The JAS weighted 

usable-habitat area percentiles for habitat summary statistics 

and time-series habitat simulations for summer drought are 

shown in figure 21. During the lowest flows in 2002, margined 

madtom and river chub habitat area were lower than the 25th

habitat percentile, whereas adult and sub-adult smallmouth 

bass habitat area were lower than the 50th percentile. In this 

reach, juvenile redbreast sunfish and sub-adult redbreast 

sunfish habitat were unaffected by low-flow conditions. 

During 1999, streamflows were less than the 

5th percentile JAS flow (262 ft3/s) for 47 days, but never 

dropped below 200 ft3/s between May 16, 1999, and 

September 5, 1999 (fig. 22). During these flow conditions, 

the habitat area decrease was not as drastic as in 2002. 

Habitat availability decreased slightly below the 50th habitat 

percentile for sub-adult smallmouth bass, margined madtom, 

and river chub in August, but did not decrease below the 

25th habitat percentile (fig. 22). No decrease in habitat area 

occurred for Cyprinella spp., smallmouth bass, juvenile 

redbreast sunfish, or sub-adult redbreast sunfish. In the 

middle section, as demonstrated by the 1999 drought, when 

flows remain above the 5th percentile JAS flow, habitat is 

not expected to decrease greatly. 

Lower Section Time-Series Scenarios

The lower section of the South Fork is the largest 

section of river, ending at the confluence of the North Fork 

and South Fork near Front Royal. The annual median flow 

for this section of river is 948 ft3/s (612.7 Mgal/d, table 2). 

The normal range of flows for JAS is 420 to 831 ft3/s (271.5 

to 537.1 Mgal/d, table 2). Combined habitat time-series plots 

for selected species or lifestages during 2002 and 1966 are 

presented (figs. 23, 24). 

Streamflows were below normal from June 1, 2002, 

to October 1, 2002; the discharge ranged from 164 to 

709 ft3/s with all but 11 days less than the 25th percentile 

flow (420 ft3/s, fig. 23). The maximum WUA for smallmouth bass, sub-adult smallmouth bass, margined madtom, 

Cyprinella spp., and river chub is associated with flows greater 

than the 25th percentile JAS flow between 450 and 557 ft3/s 

(table 19). During 2002, habitat availability decreased slightly 

for all species except juvenile redbreast sunfish and sub-adult 

redbreast sunfish. The adult smallmouth bass, sub-adult 

smallmouth bass, and margined madtom habitat area were 

lower than 25th habitat percentile, and less than the normal 

range (fig. 23). This habitat decrease occurred when flows 

were below 300 ft3/s, which occurred three times for at least 

6 days during this period. All other species of fish had habitat 

availability within the normal range for JAS. River chub 

habitat availability during 2002 had a slightly different pattern 

than the other species. As flows decreased below 450 ft3/s, 

habitat availability remained steady slightly higher than the 

25th habitat percentile. No major decreases occurred for river 

chub habitat when flows decreased below 300 or 200 ft3/s; 

however, the habitat was less than would be available within 

the normal range of flows (fig. 23, table 19).

Similar patterns are shown in time-series plots for 1966 

as flows decreased below 200 ft3/s and eventually below 

130 ft3/s (the lowest simulated flow for this section of river; 

fig. 24). The time-series plots of habitat availability for game 

and nongame fish differed with very low flows during 1966. 

The smallmouth bass and sub-adult smallmouth bass habitat 

area continued to decrease as flows decreased (fig. 24); 

however, the river chub, and margined madtom habitat area 

remained constant as flows decreased less than 300 ft3/s 

(figs. 17, 24). In combination, the WUA and time-series plots 

illustrate that as flows decrease below the 25th percentile 

JAS flow to the 5th percentile flow in the lower section of 

the South Fork, habitat availability for game fish (except 

juvenile redbreast sunfish) will continue to decrease, but 

habitat availability for nongame fish remains below normal, 

but stable. 
42 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

June July August September October

6

8

10

x 104

2

5

3

4

x 104

6

8

10

x 104

0.8

1

1.2

1.4

x 104

102

103

Discharge, in cubic

feet per second Weighted usable habitat area per 1,000 feet of stream

 100

 1,000

 200

 50

Habitat time series middle section South Fork Shenandoah River, Historic Flows, 2002

Discharge at the Luray Streamflow-Gaging Station, 2002

2002

Sub-adult smallmouth bass

Adult smallmouth bass

Margined madtom

River chub

Game fish habitat

Nongame fish habitat

EXPLANATION

A

B

C

D

E

Normal range of habitat for

 July–August–September

75% habitat percentile

50% habitat percentile

25% habitat percentile

Discharge

Figure 21. Habitat time series for the middle section of the South Fork Shenandoah River near Luray, Virginia, 

during 2002. A and B, game fish weighted usable-habitat area; C and D, nongame fish weighted usable-habitat 

area; and E, daily mean discharge for the river near Luray (01629500).
RHABSIM Modeling to Determine Fish Habitat Availability and Recreation Conditions 43

June July August September

6

8

10

x 104

2

5

3

4

x 104

6

8

10

x 104

0.8

1

1.2

1.4

x 104

102

103

Discharge, in cubic

feet per second Weighted usable habitat area per 1,000 feet of stream

 100

 1,000

 200

 50

Habitat time series middle section South Fork Shenandoah River, Historic Flows, 1999

Discharge at the Luray Streamflow-Gaging Station, 1999

1999

Sub-adult smallmouth bass

Adult smallmouth bass

Margined madtom

River chub

Game fish habitat

Nongame fish habitat

EXPLANATION

A

B

C

D

E

Normal range of habitat for

 July–August–September

75% habitat percentile

50% habitat percentile

25% habitat percentile

Discharge

Figure 22. Habitat time series for the middle section of the South Fork Shenandoah River near Luray, Virginia, 

during 1999. A and B, game fish weighted usable-habitat area; C and D, nongame fish weighted usable-habitat area; 

and E, daily mean discharge for the river near Luray (01629500).
44 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

June July August September October

6

8

10

x 104

6

8

14

10

12

x 104

2

4

6

x 104

6

7

8

9

x 104

102

103

Discharge, in cubic

feet per second Weighted usable habitat area per 1,000 feet of stream

 100

 1,000

 200

Habitat time series lower section South Fork Shenandoah River, Historic Flows, 2002

Discharge for Front Royal Streamflow-Gaging Station Flows, 2002

2002

Sub-adult smallmouth bass

Adult smallmouth bass

Margined madtom

River chub

Game fish habitat

Nongame fish habitat

EXPLANATION

A

B

C

D

E

Normal range of habitat for

 July–August–September

75% habitat percentile

50% habitat percentile

25% habitat percentile

Discharge

Figure 23. Habitat time series for the lower section of the South Fork Shenandoah River near Front Royal, Virginia, 

during 2002. A and B, game fish weighted usable-habitat area; C and D, nongame fish weighted usable-habitat area; 

and E, daily mean discharge for the river at Front Royal (01631000).
RHABSIM Modeling to Determine Fish Habitat Availability and Recreation Conditions 45

May June July August September

6

8

10

x 104

6

8

14

10

12

x 104

2

4

6

x 104

6

7

8

9

x 104

102

103

Discharge, in cubic

feet per second Weighted usable habitat area per 1,000 feet of stream

 100

 1,000

 200

Habitat time series lower section South Fork Shenandoah River, Historic Flows, 1966

Discharge for Front Royal Streamflow-Gaging Station Flows, 1966

1966

Sub-adult smallmouth bass

Adult smallmouth bass

Margined madtom

River chub

Game fish habitat

Nongame fish habitat

EXPLANATION

A

B

C

D

E

Normal range of habitat for

 July–August–September

75% habitat percentile

50% habitat percentile

25% habitat percentile

Discharge

Figure 24. Habitat time series for the lower section of the South Fork Shenandoah River near Front Royal, Virginia, 

during 1966. A and B, game fish weighted usable-habitat area; C and D, nongame fish weighted usable-habitat area; 

and E, daily mean discharge for the river at Front Royal (01631000).
46 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

Water-Withdrawal Reduction Scenario Results

Reductions in water withdrawals were simulated for 2002 

to model potential water-conservation strategies that could be 

implemented during a drought for each section of the South 

Fork (figs. 25–27). Withdrawal reductions that were simulated 

were percentages of the published 2005 withdrawal amounts 

for each section shown in table 4. Simulation results for 2002 

were fairly similar regardless of river section, so they were 

summarized together. Habitat availability for game fish was 

below normal in the 2002 time series before any withdrawal 

scenarios were considered. With 10‐ or 20‐percent reduced 

water withdrawals, minor increases in habitat availability were 

observed (figs. 25–27). For game fish, 50‐percent water-withdrawal reductions resulted in habitat availability within the 

normal range for habitat in the upper and middle river sections 

(figs. 25–26). For nongame fish such as river chub, 20‐percent 

withdrawal reductions resulted in habitat conditions within the 

normal range in the upper and middle river sections, but in the 

lower section near Front Royal, nongame habitat availability 

did not increase much with any water-withdrawal reduction 

scenario simulated for 2002 (fig. 27). In the same section, 

habitat availability for nongame fish stabilized near the 25th

habitat percentile and very little change in physical habitat 

area was simulated regardless of withdrawal scenario.

Water withdrawal scenarios were run for 2002 to assess 

canoeing habitat availability during low-flow periods. As with 

the fish habitat assessments, 10‐, 20‐, and 50‐percent reduction in water withdrawals for 2002 were simulated, and 

the resulting habitat conservation options are presented in 

figure 28A as an example of the general results for all sections. 

The 10‐ and 20‐percent reductions in withdrawals show almost 

no difference in WUA for canoeing, and the 50‐percent reduction only provides a minimal increase in canoeing suitability. 

For example, streamflows in 2002 for the upper river section 

(fig. 28) were less than 264 ft3/s most of the time and did not 

increase greater than 300 ft3/s for much of the time period. 

Because flows equal to 460 ft3/s are necessary to ensure 

passage in riffles, little gain for recreation would be expected 

even with a 50‐percent reduction of withdrawals during 

periods similar to the 2002 simulations. 

Increased Water-Use Scenario Results

Assuming that future population growth will match 

the 12‐percent growth of the previous 10 years, water use 

is expected to increase likewise in the Shenandoah Valley. 

To demonstrate the potential effects of increased water use 

from the South Fork, scenarios representing an increase in 

surface-water use were simulated at 5, 20, and 50 percent. 

As with the withdrawal reduction scenarios, one game 

species and one nongame species are presented in the figures 

to represent the effects that increased water use may have 

on habitat availability (figs. 29–31). For the upper section, 

the 50‐percent simulation results appear to show a bottom 

threshold for habitat availability; however, this is an artifact 

of the model simulating flows lower than the lowest modeled 

streamflow of 72 ft3/s for the upper reach at Lynnwood. For 

most species, habitat availability was below normal in the 

2002 time-series simulations. Increases of 5‐percent water 

withdrawals resulted in a slight reduction in habitat availability; however, 20‐ and 50‐percent withdrawals resulted in 

reductions of habitat substantially less than the 25th habitat 

percentile, or below normal for game fish in all sections of the 

river and for nongame fish in the upper and middle sections 

(figs. 29–31). When daily streamflows were higher than the 

10th percentile JAS flow, habitat availability for game fish and 

nongame fish was much less sensitive to 5‐percent increased 

water-use scenarios than when daily streamflows were less 

than the 10th percentile flow (figs. 29–31). These scenarios 

show that for normal or slightly dry years, increased water 

use is not likely to correspond with extensive habitat loss for 

game fish or nongame fish; however, during drought years, 

20‐ to 50‐percent increased water use may affect game fish 

habitat availability for all sections and nongame fish habitat 

availability in the upper and middle sections of the river. For 

nongame fish, no major reductions of habitat area were associated with increased water-use scenarios in the lower section. 

To demonstrate the potential effects of increased water 

use on recreation during drought, scenarios representing an 

increase in surface-water use were simulated at 5, 20, and 

50 percent. For canoeing suitability (fig. 28B), increases in 

water withdrawals resulted in very small decreases in available habitat, except when streamflows were already less than 

the 5th percentile flow for JAS. However, the flow range 

simulated for 2002 generally represents unsuitable conditions 

for recreation. 
RHABSIM Modeling to Determine Fish Habitat Availability and Recreation Conditions 47

June July August September October

3.5

3

2.5

2

4

4.5

5

x 104

6

5

7

10

8

9

x 104

Discharge, in cubic

feet per second Weighted usable habitat area per 1,000 feet of stream

 100

 1,000

 50

Game fish: black line = Sub-adult Smallmouth Bass habitat

Nongame fish: black line = River Chub habitat

Habitat time series upper section South Fork Shenandoah River, Historic Flows, 2002

2002

Discharge at the Lynnwood Streamflow-Gaging Station, 2002

Habitat, no reductions

10% reduced withdrawals

20% reduced withdrawals

50% reduced withdrawals

EXPLANATION

A

B

C

Normal range of habitat for

 July–August–September

75% habitat percentile

50% habitat percentile

25% habitat percentile

Discharge

Figure 25. Habitat time-series water-withdrawal reduction scenarios for the upper section of the South Fork 

Shenandoah River near Lynnwood, Virginia, during 2002. A, sub-adult smallmouth bass weighted usable-habitat 

area with 10-, 20-, and 50-percent reduction in water withdrawals; B, river chub weighted usable-habitat area 

with 10-, 20-, and 50-percent reduction in water withdrawals; and C, daily mean discharge for the river near 

Lynnwood (01628500).
48 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

May June July August September

8

4

6

10

12

x 104

0.6

1

0.8

1.4

1.2

x 105

Discharge, in cubic

feet per second Weighted usable habitat area per 1,000 feet of stream

 100

 1,000

 50

Game fish: black line = Sub-adult Smallmouth Bass habitat

Nongame fish: black line = River Chub habitat

Habitat time series water-withdrawal scenarios, middle section of the South Fork Shenandoah River, based on 2002 historic flows

2002

Discharge at the Luray Streamflow-Gaging Station, 2002

Habitat, no reductions

10% reduced withdrawals

20% reduced withdrawals

50% reduced withdrawals

EXPLANATION

A

B

C

Normal range of habitat for

 July–August–September

75% habitat percentile

50% habitat percentile

25% habitat percentile

Discharge

Figure 26. Habitat time-series water-withdrawal reduction scenarios for the middle section of the South Fork 

Shenandoah River near Luray, Virginia, during 2002. A, sub-adult smallmouth bass weighted usable-habitat area 

with 10-, 20-, and 50-percent reduction in water withdrawals; B, river chub weighted usable-habitat area with 

10-, 20-, and 50-percentreduction in water withdrawals; and C, daily mean discharge for the river near Luray 

(01629500).
RHABSIM Modeling to Determine Fish Habitat Availability and Recreation Conditions 49

June July August September October

9

7

8

10

11

x 104

6.5

7

8

7.5

9

8.5

x 104

Discharge, in cubic

feet per second Weighted usable habitat area per 1,000 feet of stream

 200

 1,000

 100

Game fish: black line = Sub-adult Smallmouth Bass habitat

Nongame fish: black line = River Chub habitat

Habitat time series water-withdrawal scenarios, lower section of the South Fork Shenandoah River, based on 2002 historic flows

2002

Discharge at the Front Royal Streamflow-Gaging Station, 2002

102

103

Habitat, no reductions

10% reduced withdrawals

20% reduced withdrawals

50% reduced withdrawals

EXPLANATION

A

B

C

Normal range of habitat for

 July–August–September

75% habitat percentile

50% habitat percentile

25% habitat percentile

Discharge

Figure 27. Habitat time-series water-withdrawal reduction scenarios for the lower section of the South Fork 

Shenandoah River near Front Royal, Virginia, during 2002. A, sub-adult smallmouth bass weighted usable-habitat 

area for 10-, 20-, and 50-percentreduction in water withdrawals; B, river chub weighted usable-habitat area with 

10-, 20-, and 50-percent reduction in water withdrawals; and C, daily mean discharge for the river at Front Royal 

(01631000).
50 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

June July August September October

1.5

0

0.5

1

2

2.5

1.5

0

0.5

1

2

2.5

104

103

102

101

Discharge, in cubic

feet per second Weighted usable habitat area per 1,000 feet of stream

 100

 10

 10,000

 1,000

 200

Canoeing habitat time series and water-withdrawal scenarios for the upper section South Fork Shenandoah River, based on 2002 historic flows, 2002

2002

Discharge at the Lynnwood Streamflow-Gaging Station, 2002

Habitat, no reductions

10% reduced withdrawals

20% reduced withdrawals

50% reduced withdrawals

Habitat, no increases

5% increased withdrawals

20% increased withdrawals

50% increased withdrawals

EXPLANATION

A

B

C

Explanation for A Explanation for B

Discharge

Figure 28. Canoeing habitat time-series water-withdrawal reduction scenarios and habitat time-series increased 

water-withdrawal scenarios for the upper section of the South Fork Shenandoah River near Lynnwood, Virginia, 

during 2002. A, canoeing weighted usable-habitat area for 10, 20, and 50 percent decrease in water withdrawals; 

B, canoeing weighted usable-habitat area for 5-, 20-, and 50-percent increase in water use; and C, daily mean 

discharge for the river near Lynnwood (01628500).
RHABSIM Modeling to Determine Fish Habitat Availability and Recreation Conditions 51

June July August September October

3

1

2

4

5

x 104

5

6

8

7

10

9

x 104

Discharge, in cubic

feet per second Weighted usable habitat area per 1,000 feet of stream

WUA WUA

 100

 1,000

 50

Game fish: black line = Sub-adult Smallmouth Bass habitat

Nongame fish: black line = River Chub habitat

Habitat time series water-withdrawals scenarios, upper section of the South Fork Shenandoah River, based on 2002 historic flows

2002

Discharge at the Lynnwood Streamflow-Gaging Station, 2002

Habitat, no increases

5% increased withdrawals

20% increased withdrawals

50% increased withdrawals

EXPLANATION

A

B

C

Normal range of habitat for

 July–August–September

75% habitat percentile

50% habitat percentile

25% habitat percentile

Discharge

Figure 29. Habitat time-series increased water-withdrawal scenarios for the upper section of the South Fork 

Shenandoah River near Lynnwood, Virginia, during 2002. A, sub-adult smallmouth bass weighted usable-habitat 

area with 5-, 20-, and 50-percent increase in water use; B, river chub weighted usable-habitat area with 5-, 20-, 

and 50-percent increase in water use; and C, daily mean discharge for the river near Lynnwood (01628500).
52 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

June July August September October

8

7

5

6

9

10

11

x 104

0.8

0.9

1

1.2

1.1

1.4

1.3

x 105

Discharge, in cubic

feet per second Weighted usable habitat area per 1,000 feet of stream

 100

 1,000

 50

Game fish: black line = Sub-adult Smallmouth Bass habitat

Nongame fish: black line = River Chub habitat

Habitat time series water-withdrawals scenarios, middle section of the South Fork Shenandoah River, based on 2002 historic flows

2002

Discharge at the Luray Streamflow-Gaging Station, 2002

Habitat, no increases

5% increased withdrawals

20% increased withdrawals

50% increased withdrawals

EXPLANATION

A

B

C

Normal range of habitat for

 July–August–September

75% habitat percentile

50% habitat percentile

25% habitat percentile

Discharge

Figure 30. Habitat time-series increased water-withdrawal scenarios for the middle section of the South Fork 

Shenandoah River near Luray, Virginia, during 2002. A, sub-adult smallmouth bass weighted usable-habitat area 

with 5-, 20-, and 50-percent increase in water use; B, river chub weighted usable-habitat area with 5-, 20-, and 

50-percent increase in water use; and C, daily mean discharge for the river near Luray (01629500).
RHABSIM Modeling to Determine Fish Habitat Availability and Recreation Conditions 53

8

7

5

6

9

10

11

x 104

6.5

7

8

7.5

9

8.5

x 104

Discharge, in cubic

feet per second Weighted usable habitat area per 1,000 feet of stream

Game fish: black line = Sub-adult Smallmouth Bass habitat

Nongame fish: black line = River Chub habitat

Habitat time series water-withdrawals scenarios, lower section of the South Fork Shenandoah River, based on 2002 historic flows

Discharge at the Front Royal Streamflow-Gaging Station, 2002

June July August September October

 200

 1,000

 100

2002

Discharge at the Front Royal Streamflow-Gaging Station, 2002

102

103

Habitat, no increases

5% increased withdrawals

20% increased withdrawals

50% increased withdrawals

EXPLANATION

A

B

C

Normal range of habitat for

 July–August–September

75% habitat percentile

50% habitat percentile

25% habitat percentile

Discharge

Figure 31. Habitat time-series and water-withdrawal scenarios for the lower section of the South Fork 

Shenandoah River near Front Royal, Virginia, during 2002. A, sub-adult smallmouth bass weighted usable-habitat 

area with 5-, 20-, and 50-percent increase in water use; B, river chub weighted usable-habitat area with 5-, 20-, 

and 50-percent increase in water use; and C, daily mean discharge for the river at Front Royal (01631000).
54 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

Summary and Conclusions

The ecological habitat requirements of aquatic organisms 

and recreational streamflow requirements of the South Fork 

Shenandoah River were investigated by the U.S. Geological 

Survey in cooperation with the Central Shenandoah Valley 

Planning District Commission, the Northern Shenandoah 

Valley Regional Commission, and Virginia Commonwealth 

University beginning in 2005. The goal was to provide the 

counties and communities in the South Fork Basin information 

about water resources in the basin, the availability of water for 

fish habitat and canoeing, and the potential effects of withdrawals and conservation measures on the ecology, recreation, 

and water supply. This investigative report completes the 

modeling for the two major tributaries of the Shenandoah 

River, providing consistent results to planning district 

personnel toward effective drought preparation in the planning 

districts that span both watersheds. A key assumption of this 

investigation is that flow is a major determinant of physical 

habitat in rivers, which in turn is a major determinant of biotic 

composition. Another assumption is that physical habitat is a 

limiting factor for fish populations. This study incorporated 

critical flow components representing the annual flow regime 

as well as the summer low-flow period during July, August, 

and September (JAS) for three streamflow-gaging stations. 

Years with flows much lower than the normal range of flows 

in JAS (typically 10th percentile or lower) were classified as 

drought years. 

Water-use data for 2005 were summarized for major 

tributaries of the South Fork, the North, Middle, and 

South Rivers for a total of 37.9 million gallons per day 

(Mgal/d). Water was withdrawn for the South Fork near 

Lynnwood (0.4 Mgal/d), Luray (3.5 Mgal/d), and Front 

Royal (2.4 Mgal/d). The cumulative total for the South Fork 

Shenandoah River surface-water withdrawals for 2005 was 

44.2 Mgal/d. During the low-flow period, the surface-water 

withdrawals represent 12 to 19 percent of the median flow.

Mesohabitat data were mapped along the entire length 

of the South Fork for use in prioritization of hydraulic 

monitoring reaches and to weight modeling results to represent the habitats within the entire length of the South Fork. 

The mesohabitat data represent moderately large hydrogeomorphic habitat units within each stream, such as a riffle, 

run, or pool having relatively homogeneous channel characteristics. The overall makeup of habitat throughout the South 

Fork is 11.7‐percent riffle, 52.6‐percent run and 35.7‐percent 

pool. These data were useful for summarizing the overall 

habitat availability for fish in the river and served as the basis 

for weighting of cross section habitat types in the habitat 

modeling phase of this study. 

Four reaches of the South Fork were studied between 

Lynnwood (located downstream from the confluence of the 

North, Middle, and South Rivers) and Front Royal (located 

near the confluence of the North Fork). Standard methods 

were used to collect channel and floodplain topographic 

information, and at least five measurements of hydrologic data 

were taken over a range of streamflows which represented 

summer flow conditions. Measurements included watersurface level, water depth, water velocity, and discharge, 

which are relevant to both recreation and fish habitat, as well 

as substrate and cover characteristics, which are relevant to 

fish habitat. 

Field-collected data were used to develop habitat 

suitability criteria (HSC) of fish in the South Fork. Optimal 

and marginal HSC were defined for fishes of the South Fork 

having observation data of 30 or larger with the exception of 

smallmouth bass, which had a sample size of only 18. Ranges 

of optimal habitat were defined as those used by the central 

50 percent of the taxa or life stage sampled. Marginal habitats 

were those used by the fish outside the range of the central 

50 percent, but within the central 80 percent of the taxa or life 

stage sampled. The HSC were developed for sub-adult and 

adult smallmouth bass (Micropterus dolomieu), juvenile and 

sub-adult redbreast sunfish (Lepomis auritus), Cyprinella spp., 

margined madtom (Noturus insignis), and river chub (Nocomis 

micropogon).

Flow and habitat modeling were completed using the 

program RHABSIM 3.0 for DOS and Windows. Fieldmeasured flow calibration datasets were combined with the 

HSC to simulate available ecological and recreational habitat 

for 30 streamflows. This resulted in development of a functional relation between habitat and streamflow and is expressed 

in the form of weighted usable-habitat area (WUA) curves.

Modeling results for usable-habitat area for each fish 

species and canoeing suitability were compared to critical flow 

statistics, such as the normal range of flows, the 5th percentile 

flow for JAS flows, and the annual 7Q10 statistic to determine 

flows when habitat may be limited. In the Lynnwood section 

(upper section) the normal range of flows for JAS includes the 

maximum usable-habitat area for sub-adult redbreast sunfish, 

adult smallmouth bass, and sub-adult smallmouth bass. Lower 

than normal flows, however, provide greater habitat availability for smaller riffle-dwelling fish like Cyprinella spp., 

river chub, margined madtom, and juvenile redbreast sunfish. 

The Lynnwood study reach contains one riffle which could 

limit canoe passage, but model results indicate that a discharge 

of 460 cubic feet per second (ft3/s) could ensure that depths in 

the riffle are greater than or equal to 1‐foot (ft) deep, sufficient 

for canoe passage. For ideal canoeing conditions, modeling 

results for Lynnwood indicate a discharge of 604 ft3/s up to 

2,400 ft3/s. 

For the Luray section (middle section) the normal range 

of flows for JAS includes the maximum usable habitat for all 

fish, except juvenile redbreast sunfish and sub-adult redbreast 

sunfish, which have maximum usable habitat lower than the 

5th percentile flow and slightly lower than the 25th percentile 

flow, respectively. Despite the large range of maximum usable 

habitat among species, the 10th percentile flow (300 ft3/s) 

seems to maintain habitat availability for all species in the 

middle section. Kauffman Mill study reach has a high-gradient 

riffle that could limit canoe passage, but model results indicate 
Summary and Conclusions 55

that a discharge equal to 520 ft3/s should provide an average 

depth of 1.3 ft, sufficient for canoe passage. For ideal canoeing 

conditions, modeling results for Luray indicate a discharge of 

890 ft3/s. 

For the Front Royal (lower section) the normal range of 

flows for JAS includes the maximum usable habitat for all 

fish, except juvenile redbreast sunfish and sub-adult redbreast 

sunfish, which have maximum usable habitat lower than the 

5th percentile flow and slightly lower than the 25th percentile 

flow, respectively. For pool and run-dwelling game fish like 

smallmouth bass and redbreast sunfish, habitat availability 

does not vary much from the normal range of flows for the 

low-flow period, down to the 5th percentile flow. Below the 

5th percentile, habitat sharply decreases. For riffle-dwelling 

fish, the normal range of flows maintains habitat availability, 

and below the 25th percentile flow habitat remains below 

normal and steady. Canoeing through the Front Royal section 

takes the paddler through the widest channels on the South 

Fork and extensive sections of bedrock riffle like that found 

along the Thunderbird Farms study reach. Recreational habitat 

modeling results indicate that ideal canoeing conditions range 

from 831 up to 2,064 ft3/s. 

Habitat availability can be further analyzed by developing 

an association between habitat and streamflow-gaging stations 

on the South Fork that were in operation during previous 

droughts. This analysis should help identify species and 

habitats affected by drought. The results are interpretations of 

the weighted usable-habitat area curves and the historic flow 

record to create “time-series plots” of habitat and streamflow. 

The time-series associations can be ranked for the entire 

period of record for a streamgage to create a historic summary 

of habitat with a focus on the months of JAS. This allows 

the evaluation of daily habitat time-series data for a summer 

drought within the context of the normal habitat range, or the 

25th to 75th percentile of habitat. Multiple historic summer 

drought low-flow periods were examined for each study site 

when data were available; the most extreme drought on record 

occurred in 2002, which represents worst-case conditions for 

each section. 

Time-series results for drought years such as 2002 

showed that extreme low-flow conditions less than the 

5th percentile of flow for JAS corresponded to below-normal 

habitat availability for both game and nongame fish in the 

upper section of the river. During 2002, habitat availability 

for all species except juvenile redbreast sunfish decreased 

between 40 to 70 percent for a given species when flows 

decreased from 200 to 84 ft3/s. For the middle section near 

Luray, time-series results from 2002 showed habitat area was 

below normal for nongame fish, such as margined madtom 

and river chub, whereas habitat area for game fish adult and 

sub-adult smallmouth bass remained within normal range. 

For comparison in 1999, habitat availability decreased 

slightly less than the median habitat percentile for sub-adult 

smallmouth bass, margined madtom, and river chub, but did 

not decrease below normal during that year’s drought. In the 

middle section, as demonstrated by the 1999 drought, when 

flows remain above the 5th percentile JAS flow, habitat availability is not expected to decrease outside the normal range 

of habitat. In the lower section near Front Royal, time-series 

results for adult smallmouth bass, sub-adult smallmouth bass, 

and margined madtom habitat for 2002 were below normal 

when flows were below the 10th percentile JAS flow. All other 

species of fish had habitat availability within the normal range 

for JAS.

Adequate flows for canoeing rarely occurred during 

the 2002 low-flow period. For instance, along the middle 

section of the South Fork, flows less than the 25th percentile 

flow for JAS did not provide adequate depth for passage 

through riffle habitats. Although recreation is certainly a 

consideration for water-resources management, when flows 

were far below the 25th percentile and decreasing, time-series 

results did not indicate that paddling conditions could be 

favorable even with conservation efforts, such as reduced 

water withdrawals. 

 Water-conservation scenarios representing 10‐ or 

20‐percent reduced water withdrawals resulted in minor 

increases in habitat availability for game fish for the 2002 

drought simulation. For game fish, 50‐percent waterwithdrawal reduction resulted in habitat availability within the 

normal range for habitat in upper and middle river sections. 

For nongame fish such as river chub, a 20‐percent withdrawal 

reduction resulted in habitat availability within the normal 

range for habitat in the upper and middle river sections. 

Water-withdrawal reduction scenarios simulated for 2002 for 

the lower section near Front Royal did not result in increased 

habitat availability for nongame fish. 

Increased water-use scenarios representing a 5‐percent 

increase in water withdrawals resulted in a slight reduction in 

habitat availability for the 2002 drought simulation; however, 

20‐ and 50‐percent increased withdrawals resulted in habitat 

availability substantially less than the 25th habitat percentile, 

or below normal. Habitat reductions were more pronounced 

when flows were lower than the 10th percentile flow for 

JAS. The results show that for normal or slightly dry years, 

increased water withdrawals are not likely to correspond 

with extensive habitat loss for game fish or nongame fish; 

however, during drought years, 20‐ to 50‐percent increased 

water withdrawals may result in below normal habitat availability for game fish throughout the river and nongame fish in 

the upper and middle sections of the river. These simulations 

of rare historic drought conditions, such as those observed in 

2002, serve as a baseline for development of ecological flow 

thresholds for drought planning. 
56 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

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Appendixes
60 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

0

0.5

1

0 2 4 6 0 2 4 6 8

0

0.5

1

0 1 2 3 0 1 2 3 4 5

0 1 2 3

0

0.5

1

0 1 2 3 4

0

0.5

1

0 1 2 3 4

Depth, in feet

Depth, in feet

Habitat suitability rank

Sub-adult smallmouth bass Adult smallmouth bass

Juvenile redbreast sunfish Sub-adult redbreast sunfish

Margined madtom

River chub

Cyprinella spp.

Appendix 1. Fish habitat suitability 

criteria for depth, velocity, and substrate 

characteristics on the south Fork 

Shenandoan River, Virginia. Substrate 

codes are listed in table 8.

0

0.5

1

0 0.5 1 1.5 2 0 0.5 1 1.5

0

0.5

1

0 0.5 1 1.5 0 0.5 1 1.5 2

0 0.5 1 1.5 2 2.5

0

0.5

1

0 1 2 3 4

0

0.5

1

0 0.5 1 1.5 2 2.5

Velocity, in feet per second

Velocity, in feet per second

Habitat suitability rank

Sub-adult smallmouth bass Adult smallmouth bass

Juvenile redbreast sunfish Sub-adult redbreast sunfish

Margined madtom

River chub

Cyprinella spp.
Appendixes 61

0

0.5

1

0 2 4 6 8 10 12

0 2 4 6 8 10 12

0 2 4 6 8 10 12

0 2 4 6 8 10 12

0 2 4 6 8 10 12

0 2 4 6 8 10 12

0

0.5

1

0

0.5

1

0 2 4 6 8 10

0

0.5

1

Substrate code

Substrate code

Habitat suitability rank

Sub-adult smallmouth bass Adult smallmouth bass

Juvenile redbreast sunfish Sub-adult redbreast sunfish

Margined madtom

River chub

Cyprinella spp.

Appendix 1. Fish habitat suitability 

criteria for depth, velocity, and substrate 

characteristics on the south Fork 

Shenandoan River, Virginia. Substrate 

codes are listed in table 8.—Continued
62 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...

Appendix 2. River habitat simulation (RHABSIM) model-calibration data from the hydraulic data-collection reaches on the South Fork 

Shenandoah River, Virginia.

[Calculated discharge is an RHABSIM calculated value. SZF, stage zero flow; WSL, water-surface level; slope, average water-surface slope for 

all discharges measured; verticals, number of measurement points along the transect; ND, no data collected]

Transect

Observed 

WSL,1

feet

Simulated 

WSL, 

feet

Reach 

calibration 

discharge,1

ft3/s

Modelcalculated 

discharge,

ft3/s

Verticals

Transect 

weighting 

factor

Slope,1

percent

SZF,1

feet

Average 

depth, 

feet

Wetted 

width, 

feet

Velocity 

mean,

 ft/s

Lynnwood

1,015.03 1,015.04 181 177.15 1.36 168.20 0.67

1,015.12 1,015.12 221 228.66 1.42 169.77 0.86

1 1,015.78 1,015.79 725 769.61 35 0 0.0039 1,012.4 1.91 188.20 1.84

1,015.89 1,015.88 780 809.88 1.93 196.50 1.74

1,016.36 1,016.36 1,132 1,331.46 2.32 204.62 2.44

1,014.28 1,014.28 181 140.94 0.65 137.93 1.13

1,014.35 1,014.35 221 159.66 0.71 141.12 1.25

2 1,015.31 1,015.31 725 665.00 33 0.25 0.00340 1,012.31 1.51 162.41 2.46

1,015.38 1,015.38 780 748.39 1.56 163.99 2.69

1,015.86 1,015.86 1,132 1,363.80 1.93 175.13 3.80

1,014.11 1,014.02 181 273.50 1.19 166.91 0.88

1,014.13 1,014.13 221 278.99 1.20 167.05 0.93

3 1,014.82 1,014.89 725 851.86 37 0.5 0.00100 1,011.58 1.84 172.20 2.08

1,014.96 1,014.95 780 948.66 1.98 172.99 2.21

1,015.41 1,015.27 1,132 1,352.33 2.40 175.52 2.78

1,014.03 1,013.94 181 188.28 1.76 183.49 0.58

1,014.04 1,014.04 221 237.00 1.78 183.68 0.72

4 1,014.74 1,014.80 725 774.42 36 0.62 0.00040 1,011.57 2.39 191.27 1.59

1,014.86 1,014.85 780 814.84 2.49 192.52 1.61

1,015.29 1,015.15 1,132 1,250.16 2.87 196.35 2.04

1,014.01 1,013.93 181 181.17 1.74 169.99 0.62

1,014.03 1,014.03 221 198.69 1.75 170.15 0.68

5 1,014.74 1,014.78 725 748.76 31 1 0.00040 1,010.81 2.18 195.36 1.53

1,014.84 1,014.83 780 841.49 2.27 195.69 1.67

1,015.26 1,015.12 1,132 1,225.79 2.67 197.17 2.08

Riverbend Pool

905.13 905.12 271 262.99 5.69 243.41 0.15

905.47 905.45 380 284.01 6.00 245.00 0.15

1 905.79 905.80 554 553.97 29 0 0.00005 895.58 6.28 246.47 0.31

906.12 906.13 962 1,142.06 6.57 247.97 0.61

906.51 906.51 1,187 1,308.75 6.91 249.79 0.56

905.12 905.12 271 240.65 7.98 218.77 0.11

905.46 905.45 380 303.92 8.24 220.89 0.14

2 905.79 905.80 554 564.91 31 0.2 0.00004 893.30 8.49 223.00 0.26

906.11 906.13 962 1,005.27 8.74 225.03 0.50

906.50 906.51 1,187 1,073.40 9.02 227.80 0.52

905.12 905.12 271 191.85 8.33 205.60 0.10

905.44 905.44 380 269.30 8.55 208.16 0.15

3 905.79 905.79 554 609.68 30 0.6 0.00004 890.50 8.78 210.92 0.29

906.11 906.11 962 1,018.35 9.00 213.46 0.49

906.49 906.49 1,187 1,238.59 9.24 216.41 0.58

Kauffman Mill

723.02 723.01 358 ND2 2.31 216.93 ND2

723.27 723.27 631 ND2 2.52 219.80 ND2

0 723.50 723.50 890 923.20 34 0 0.0032 720.54 2.72 222.57 1.52

723.93 723.93 1,540 ND2 2.98 235.67 ND2

724.21 724.21 2,087 2,193.99 3.18 242.73 2.48
Appendixes 63

Transect

Observed 

WSL,1

feet

Simulated 

WSL, 

feet

Reach 

calibration 

discharge,1

ft3/s

Modelcalculated 

discharge,

ft3/s

Verticals

Transect 

weighting 

factor

Slope,1

percent

SZF,1

feet

Average 

depth, 

feet

Wetted 

width, 

feet

Velocity 

mean,

 ft/s

Kauffman Mill—Continued

722.43 722.43 358 489.61 1.02 288.30 1.54

722.90 722.90 631 974.65 1.47 292.01 2.23

1 723.12 723.12 890 1,484.73 49 0.11 0.0036 720.54 1.68 294.65 2.80

723.60 723.60 1,540 ND2 2.00 318.91 ND2

723.82 723.82 2,087 ND2 2.19 324.76 ND2

721.47 721.40 358 332.82 1.44 307.57 0.73

721.70 721.70 631 609.83 1.67 308.39 1.19

2 721.91 721.90 890 880.80 46 0.67 0.0052 718.61 1.88 309.15 1.58

722.28 722.27 1,540 1,879.31 2.23 310.48 2.77

722.65 722.51 2,087 2,460.99 2.60 311.82 3.08

720.64 720.58 358 322.18 2.01 231.00 0.67

720.88 720.88 631 767.82 2.22 234.73 1.40

3 720.99 721.08 890 1,102.87 37 0.75 0.004 717.33 2.30 236.60 1.94

721.42 721.42 1,540 1,814.10 2.66 243.88 2.60

722.04 721.62 2,087 2,439.50 3.21 249.86 2.90

720.54 720.48 358 298.37 2.15 216.53 0.59

720.79 720.79 631 739.99 2.36 220.55 1.29

4 720.94 720.99 890 1,067.11 40 1 0.0016 716.8 2.38 232.76 1.58

721.34 721.34 1,540 1,745.59 2.73 237.13 2.32

721.95 721.55 2,087 2,338.24 3.26 243.75 2.64

Thunderbird Farms

494.42 494.42 327 328.17 4.47 305.22 0.23

494.46 494.44 349 406.40 4.50 305.47 0.28

1 494.46 494.47 404 408.64 49 0 0.0002 492.73 4.50 305.47 0.29

495.17 495.17 1,123 ND2 5.15 309.76 0.00

495.96 495.96 2,064 2,156.24 5.88 312.76 1.09

494.42 494.41 327 340.71 5.79 306.70 0.18

494.46 494.44 349 382.01 5.83 306.82 0.20

2 494.47 494.47 404 ND2 41 0.0 0.00010 492.73 5.84 306.85 0.00

495.17 495.16 1,123 1,168.77 6.49 309.00 0.55

495.93 495.95 2,064 2,145.55 7.20 311.34 0.91

494.42 494.41 327 309.95 2.20 364.05 0.37

494.44 494.43 349 390.91 2.22 364.22 0.47

3 494.47 494.46 404 394.86 53 0.8 0.00020 492.73 2.25 364.48 0.46

495.12 495.13 1,123 1,161.46 2.86 370.04 1.08

495.87 495.88 2,064 2,155.15 3.57 374.89 1.57

494.28 494.28 327 387.19 0.83 370.07 1.24

494.32 494.32 349 354.54 0.87 370.52 1.06

4 494.36 494.36 404 374.90 55 1 0.0009 492.73 0.91 370.98 1.14

494.97 494.97 1,123 1,212.53 1.50 377.11 2.12

495.66 495.66 2,064 2,307.17 2.16 383.79 2.78

1Value collected from field measurements. 

2No velocity dataset collected for this water-surface level.

Appendix 2. River habitat simulation (RHABSIM) model-calibration data from the hydraulic data-collection reaches on the South Fork 

Shenandoah River, Virginia.—Continued

[Calculated discharge is an RHABSIM calculated value. SZF, stage zero flow; WSL, water-surface level; slope, average water-surface slope for 

all discharges measured; verticals, number of measurement points along the transect; ND, no data collected]
64 South Fork Shenandoah River Habitat-Flow Modeling to Determine Ecological and Recreational Characteristics...
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Krstolic and Ramey—South Fork Shenandoah River Habitat Flow Modeling—Scientific Investigations Report 2012–5081

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