October 2, 1921, view of the ferry crossing the Colorado River at Lees Ferry, 

Arizona. Photograph taken by Roger C. Rice (U.S. Geological Survey) from the 

ferry landing on the right bank. Source of this photograph: 1921–37 Surface Water 

Records File, Colorado River at Lees Ferry, Arizona, file stored at the Federal 

Records Center in Laguna Niguel, California, in Accession No. 57-78-0006, 

Box 2 of 2 , Location No. MB053635.
Computation and Analysis of the 

Instantaneous-Discharge Record for the 

Colorado River at Lees Ferry, Arizona—

May 8, 1921, through September 30, 2000

By David J. Topping, John C. Schmidt, and L.E. Vierra, Jr.

PROFESSIONAL PAPER 1677
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Reston, Virginia 2003

Copies of this report can be purchased from:

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225-0286

Library of Congress Cataloging-in-Publication Data

Topping, David, J.

Computation and analysis of the instantaneous-discharge for the Colorado River at Lees

Ferry, Arizona: May 8, 1921, through September 30, 2000 / by David J. Topping, John

C. Schmidt, and L.E. Vierra, Jr.

p. cm. -- (U.S. Geological Survey professional paper; 1677)

Includes bibliographic references.

ISBN 0-607-92248-6 (alk. paper)

1. Stream measurements--Arizona--Lees Ferry Region. 2. Stream

measurements--Colorado River (Colo.-Mex.) 3. Floods--Arizona--Lees Ferry Region 4.

Floods--Colorado River (Colo.-Mex.) I. Schmidt, John C., II. Vierra, L.E., Jr. III.

Title. IV. Series.

GB1225.A6T67 2003

551.48′3′0979133--dc22 2003049324
Contents III

CONTENTS

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

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

Purpose and Scope.................................................................................................................................................... 7

Acknowledgments.................................................................................................................................................... 7

Physical Characteristics of the Lees Ferry Gaging Reach................................................................................................ 7

Stage and Discharge Measurements in the Lees Ferry Reach ............................................................................... 8

The Hydraulic Control and its Effect on the Shape of the Stage-Discharge Rating Curve.................................. 8

Pre-Dam Hysteresis in the Water-Surface Slope in the Reach Upstream from the Lees Ferry Gage ................. 15

Colorado River Floods at Lees Ferry ................................................................................................................................ 17

Estimate of the Peak Discharge of the 1921 Flood at Lees Ferry.......................................................................... 18

The Key Assumption of J.S. Gatewood and R.S. Hunter............................................................................. 18

Other Measurements of the Peak Discharge of the 1921 Flood Made Downstream and 

Upstream from Lees Ferry ............................................................................................................... 19

Revised Estimate of the Peak Discharge of the 1921 Flood at Lees Ferry.................................................. 27

Estimate of the Peak Discharge of the 1884 Flood at Lees Ferry.......................................................................... 29

The Original U.S. Geological Survey Estimate of the Peak Discharge of the 1884 Flood ........................ 29

Revised Estimate of the Peak Discharge of the 1884 Flood at Lees Ferry.................................................. 31

Implications for Previous Estimates of Paleoflood Discharges ............................................................................. 31

Computation of the Continuous Record of Instantaneous Discharge at Lees Ferry for Water Years 

1921–2000............................................................................................................................................................ 33

Methods Used by the U.S. Geological Survey to Compute the Previously Published Daily 

Mean Discharge Record for Water Years 1921–86.................................................................................. 33

Methods Used in this Study to Compute the Continuous Record of Instantaneous Discharge for 

Water Years 1921–86................................................................................................................................. 37

The Continuous Record of Instantaneous Discharge for Water Years 1921–2000.............................................. 39

Analysis of the Continuous Record of Instantaneous Discharge..................................................................................... 42

Flow Duration, Sub-Daily Discharge Variability, and Sediment Transport during the 

Pre-Dam Period........................................................................................................................................... 42

Effects of the Operation of Glen Canyon Dam on Flow Duration, Sub-Daily Discharge 

Variability, and Sediment Transport.......................................................................................................... 45

Flood Frequency during the Pre-Dam Period ......................................................................................................... 52

Effect of the Operation of Glen Canyon Dam on Flood Frequency...................................................................... 54

Conclusions......................................................................................................................................................................... 56

References Cited................................................................................................................................................................. 58

Appendix A: List of Accession, Box, and Location Numbers where the U.S. Geological Survey 

Data Used in this Study are Stored in the Federal Records Centers ................................................................. 61

Appendix B: Important Events in the History of the Lees Ferry Gaging Station........................................................... 65

Appendix C: December 30, 1929, Memorandum of W.E. Dickinson............................................................................ 85

Appendix D: Relationship between the Datums of the LaRue and Number 1 Gages.................................................... 89

Appendix E: Days with Disagreement Greater than or Equal to 5 Percent between the Daily Mean 

Discharge Computed from the Continuous Record of Instantaneous Discharge and the 

Published Daily Mean Discharge........................................................................................................................ 97

Appendix F: Monthly Flow-Duration Curves Computed from the Continuous Record of 

Instantaneous Discharge...................................................................................................................................... 109

Appendix G: Monthly Exceedance Curves of the Daily Range in Discharge Computed from the 

Continuous Record of Instantaneous Discharge ................................................................................................ 115
IV Contents

FIGURES

1. Maps of the Colorado River drainage basin and Lees Ferry........................................................................ 3

2, 3. Photographs showing:

2. The Lees Ferry Gage.............................................................................................................................. 9

3. The Upper and Lower Cableways......................................................................................................... 10

4. Graphs showing the flattening water-surface profiles with increasing stage and the reversal 

in curvature of the stage-discharge rating curve ........................................................................................... 12

5. Photographs showing flow over the gravel bar at the mouth of the Paria River at a range of 

discharges........................................................................................................................................................ 14

6. Graph showing water-surface slopes as a function of stage......................................................................... 16

7. Graph comparing the stage-discharge rating curve defined by the discharge measurements 

made during water-years 1921–22 with the revised stage-discharge rating curve of Gatewood 

and Hunter....................................................................................................................................................... 20

8. Graphs showing the stage-discharge rating curves at the Lees Ferry Gage defined by discharge 

measurements during the 6 high-discharge water years with substantial data above a 

stage of 17 feet................................................................................................................................................ 21

9, 10. Photographs showing:

9. The high-water marks from the 1921 flood and graph used to estimate the peak discharges 

of the 1921 and 1884 floods at the Grand Canyon gaging station....................................................... 22

10. A discharge measurement being made by the U.S. Reclamation Service on the morning of 

June 27, 1921, at the Yuma gaging station, near the peak of the June 1921 flood............................. 25

11. Graphs comparing the May through June daily mean discharge of the San Juan River with the 

combined previous day's discharge of the Animas and Florida Rivers during water years 

1928–37........................................................................................................................................................... 26

12, 13. Graphs showing:

12. Linear and power-law extrapolations of the stage-discharge rating curves at the Lees Ferry, 

Number 4, Cable, and Number 1 Gages............................................................................................... 27

13. The stage-discharge rating curve used in this study for the period between May 8, 1921, 

and June 25, 1921................................................................................................................................... 30

14. Photographs showing the River-mile 233.7 driftwood deposit.................................................................... 32

15. Graph showing the number of discharge measurements made at Lees Ferry during each water 

year between 1921 and 1986.......................................................................................................................... 33

16. Graph and photographs showing the pre-dam effects of ice at the Lees Ferry gaging station................... 36

17. Graph showing the number of days when the daily mean discharge record at Lees Ferry was 

modified on the basis of the discharge record at the Grand Canyon gaging station................................... 37

18. Graph comparing the computed instantaneous discharge for April–August 1921 at Lees 

Ferry with the discharge at upstream and downstream gaging stations ...................................................... 38

19–23. Graphs showing:

19. An example of the cause of an error in the published daily mean discharge record .......................... 40

20. The percent disagreement between the daily mean discharges computed from 

the continuous record of instantaneous discharge from this study and the published 

daily mean discharges............................................................................................................................ 41

21. The (A) continuous record of the instantaneous discharge and the (B) daily range in the 

discharge of the Colorado River at Lees Ferry from May 8, 1921, through 

September 30, 2000................................................................................................................................ 42

22. Flow-duration curves for the Colorado River at Lees Ferry................................................................ 44

23. The pre-dam monthly median, minimum, and maximum discharges of the Colorado 

River at Lees Ferry................................................................................................................................. 44
Contents V

24. Graph comparing the pre-dam flow-duration curve computed from instantaneous discharges 

with the pre-dam flow-duration curve computed from daily mean discharges........................................... 45

25, 26. Graphs showing:

25. The exceedance curves of the daily range in the discharge of the Colorado River at 

Lees Ferry............................................................................................................................................... 46

26. The pre-dam monthly median, minimum, and maximum daily ranges in the discharge 

of the Colorado River at Lees Ferry...................................................................................................... 47

27. Probability graph showing the pre-dam daily range in the discharge of the Colorado River at 

Lees Ferry ....................................................................................................................................................... 47

28–30. Graphs showing:

28. An example of large dam-induced daily fluctuations in the discharge of the Colorado 

River at Lees Ferry................................................................................................................................. 48

29. Flow-duration curves for the Colorado River at Lees Ferry from each of the four post-dam 

decades and the entire pre-dam period.................................................................................................. 48

30. The effects of dam operations on the monthly median, minimum, and maximum discharges 

of the Colorado River at Lees Ferry...................................................................................................... 49

31. Graph comparing the post-dam flow-duration curve computed from instantaneous discharges 

with the post-dam flow-duration curve computed from daily mean discharges......................................... 50

32, 33. Graphs showing:

32. The exceedance curves of the daily range in the discharge of the Colorado River at 

Lees Ferry for the four post-dam decades and the entire pre-dam period of record........................... 50

33. The effects of dam operations on the monthly median, minimum, and maximum daily 

ranges in the discharge of the Colorado River at Lees Ferry............................................................... 51

34. Probability graph showing the daily range in the discharge of the Colorado River at Lees Ferry 

during the pre- and post-dam periods of record............................................................................................ 51

35–38. Graphs showing:

35. The minimum and maximum discharges of the Colorado River at Lees Ferry each 

month from May 1921 through September 2000................................................................................. 52

36. The partial-duration and annual flood-frequency analyses for the pre-dam Colorado 

River at Lees Ferry................................................................................................................................. 53

37. The effect of dam operations on flood frequency on the Colorado River at Lees Ferry.................... 55

38. The duration of periods of sustained discharge over 18,500 cubic feet per second ........................... 56

B1–B7. Photographs showing:

B1. The repairs to the Upper Cableway and the high-water marks from the 1921 flood ......................... 69

B2. The Dugway Gage and the high-water mark from the 1921 flood...................................................... 72

B3. The Number 4 Gage and the high-water marks from the 1921 flood ................................................. 74

B4. The Number 1 Gage............................................................................................................................... 77

B5. The right- to left-bank view of the Upper Cableway and the Cable Gage on the 

left bank .................................................................................................................................................. 78

B6. The March 1933 downstream view of the Lees Ferry Gage and the high level of the rock 

fill placed behind the gage in 1929–30 ................................................................................................. 80

B7. The November 13, 2000, downstream view of the remnants of the Lower Staff Gage..................... 82

F1, G1. Graphs showing:

F1. The monthly flow-duration curves for the pre- and post-dam periods of record for 

the Colorado River at Lees Ferry .......................................................................................................... 111

G1. The monthly exceedance curves of the daily range in the discharge of the Colorado 

River at Lees Ferry for the pre- and post-dam periods......................................................................... 117
VI Conversion Factors and Vertical and Horizontal Datums

TABLES

1. U.S. Geological Survey gaging stations used in this study.......................................................................... 6

2. Stage-discharge rating curves used to construct the published record of daily mean 

discharge at the Lees Ferry gaging station during water years 1921–86..................................................... 34

3. Dates when the shifting-control method was used to construct the published record of daily 

mean discharge at the Lees Ferry gaging station during water years 1921–86........................................... 35

CONVERSION FACTORS AND VERTICAL AND HORIZONTAL DATUMS

CONVERSION FACTORS

In this paper, all measurements are reported in the inch-pound system to maintain consistency with historical and current U.S. Geological Survey data. Unpublished raw data and historical memoranda presented in 

this paper refer to stage in feet and discharge in cubic feet per second, and historical photographs show staff 

gages marked in feet. For readers who wish to convert stage, discharge, and water volume from the inchpound system of units to the metric system of units, the conversion factors are listed below:

VERTICAL AND HORIZONTAL DATUMS

Sea Level—In this paper, vertical coordinate information is referenced to the National Geodetic Vertical 

Datum of 1929 (NGVD1929). Thus, the term "sea level" refers to NGVD1929. Horizontal coordinate 

information is referenced to the North American Datum of 1983 (NAD83). 

Multiply By To obtain

foot (ft) 0.3048 meter (m)

mile 1.609 kilometer (km)

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

acre-foot 1,235 cubic meter (m3)

ton, short (2,000 pounds) 0.9074 metric ton (t)
Abstract 1

Computation and Analysis of the 

Instantaneous-Discharge Record for the 

Colorado River at Lees Ferry, Arizona—

May 8, 1921, through September 30, 2000

By David J. Topping, John C. Schmidt, and L.E. Vierra, Jr.

ABSTRACT

A gaging station has been operated by the U.S. 

Geological Survey at Lees Ferry, Arizona, since May 

8, 1921. In March 1963, Glen Canyon Dam was 

closed 15.5 miles upstream, cutting off the upstream 

sediment supply and regulating the discharge of the 

Colorado River at Lees Ferry for the first time in 

history. To evaluate the pre-dam variability in the 

hydrology of the Colorado River, and to determine 

the effect of the operation of Glen Canyon Dam on 

the downstream hydrology of the river, a continuous 

record of the instantaneous discharge of the river at 

Lees Ferry was constructed and analyzed for the 

entire period of record between May 8, 1921, and 

September 30, 2000. This effort involved retrieval 

from the Federal Records Centers and then synthesis 

of all the raw historical data collected by the U.S. 

Geological Survey at Lees Ferry. As part of this 

process, the peak discharges of the two largest 

historical floods at Lees Ferry, the 1884 and 1921 

floods, were reanalyzed and recomputed. This 

reanalysis indicates that the peak discharge of the 

1884 flood was 210,000±30,000 cubic feet per 

second (ft3/s), and the peak discharge of the 1921 

flood was 170,000±20,000 ft3/s. These values are 

indistinguishable from the peak discharges of these 

floods originally estimated or published by the U.S. 

Geological Survey, but are substantially less than the 

currently accepted peak discharges of these floods. 

The entire continuous record of instantaneous 

discharge of the Colorado River at Lees Ferry can 

now be requested from the U.S. Geological Survey 

Grand Canyon Monitoring and Research Center, 

Flagstaff, Arizona, and is also available electronically 

at http://www.gcmrc.gov. This record is perhaps the 

longest (almost 80 years) high-resolution (mostly 

15- to 30-minute precision) times series of river 

discharge available. Analyses of these data, therefore, 

provide an unparalleled characterization of both the 

natural variability in the discharge of a river and the 

effects of dam operations on a river.

Following the construction and quality-control 

checks of the continuous record of instantaneous 

discharge, analyses of flow duration, sub-daily flow 

variability, and flood frequency were conducted on 

the pre- and post-dam parts of the record. These 

analyses indicate that although the discharge of the 

Colorado River varied substantially prior to the 

closure of Glen Canyon Dam in 1963, operation of 

the dam has caused changes in discharge that are 

more extreme than the pre-dam natural variability. 

Operation of the dam has eliminated flood flows and 

base flows, and thereby has effectively "flattened" the 

annual hydrograph. Prior to closure of the dam, the 

discharge of the Colorado River at Lees Ferry was 

lower than 7,980 ft3/s half of the time. Discharges 

lower than about 9,000 ft3/s were important for the 

seasonal accumulation and storage of sand in the predam river downstream from Lees Ferry. The current 

operating plan for Glen Canyon Dam no longer 

allows sustained discharges lower than 8,000 ft3/s to 

be released. Thus, closure of the dam has not only cut 

off the upstream supply of sediment, but operation of 
2 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

the dam has also largely eliminated discharges during 

which sand could be demonstrated to accumulate 

in the river. In addition to radically changing the 

hydrology of the river, operation of the dam for 

hydroelectric-power generation has introduced large 

daily fluctuations in discharge. During the pre-dam 

era, the median daily range in discharge was only 

542 ft3/s, although daily ranges in discharge 

exceeding 20,000 ft3/s were observed during the 

summer thunderstorm season. Relative to the predam period of record, dam operations have increased 

the daily range in discharge during all but 0.1 percent 

of all days. The post-dam median daily range in 

discharge, 8,580 ft3/s, exceeds the pre-dam median 

discharge of 7,980 ft3/s. Operation of the dam has 

also radically changed the frequency of floods on 

the Colorado River at Lees Ferry. The frequency 

of floods with peak discharges larger than about 

29,000 ft3/s has greatly decreased, while the 

frequency of smaller floods, with peak discharges 

between 18,500 and 29,000 ft3/s, has increased 

substantially. Operation of the dam has greatly 

extended the duration of smaller floods; for example, 

each of the four longest periods of sustained flows in 

excess of 18,500 ft3/s occurred after closure of the 

dam.

INTRODUCTION

Lees Ferry on the Colorado River was initially 

chosen by the U.S. Geological Survey (USGS) as a 

measurement site because it was strategically located with 

respect to the hydrology of the Colorado River drainage 

basin and was readily accessible (Rusho and Crampton, 

1992; Reilly, 1999). Lees Ferry was readily accessible by 

automobile in the 1920s, and was the first point where the 

combined runoff could be measured from the upper part 

of the Colorado River drainage basin, which includes the 

upper Colorado, Green, and San Juan Rivers (fig. 1A). 

Lees Ferry also was chosen as the location for a gaging 

station because it was several miles downstream from a 

proposed dam site in lower Glen Canyon favored by the 

Southern California Edison Company, the cooperator who 

maintained this gaging station for the first several years.

Stage of the Colorado River at Lees Ferry, Arizona, 

has been measured since May 8, 1921, when the first staff 

gage was installed by E.C. LaRue of the USGS. 

Subsequently, stage was read at least twice daily on 

several or more staff gages installed in the Lees Ferry 

reach. Discharge has been measured at Lees Ferry since 

August 3, 1921, when the first cableway across the river 

was completed. Stage of the Colorado River at Lees Ferry 

has been measured continuously since January 19, 1923, 

when a permanent recording stage gage, consisting of a 

strip-chart recorder connected to a float assembly housed 

in a concrete stilling well, became operational. This 

recording stage gage is the modern gage and is located in 

Glen Canyon National Recreation Area, 15.5 miles 

downstream from Glen Canyon Dam, and 1 mile 

upstream from the mouth of the Paria River and the 

northeastern boundary of Grand Canyon National Park 

(fig. 1B). In this paper, the recording stage gage housed in 

the concrete stilling well is referred to as the Lees Ferry 

Gage. The historical staff gages in the reach (fig. 1C) and 

the Lees Ferry Gage together are referred to as the Lees 

Ferry gaging station. The official USGS station names and 

numbers for the Lees Ferry and other gaging stations used 

in this study, and the shortened names used in this paper 

are listed in table 1. 

The Lees Ferry gaging station has been 

continuously maintained since 1921 because the site is 

also strategically located in a political sense. One year 

after the establishment of this gaging station, the 1922 

Colorado River Compact was negotiated between the 

seven states in the Colorado River drainage basin. The 

Compact divided the drainage basin into two parts: the 

Upper Basin and the Lower Basin (fig. 1A). In the 

Compact, the dividing point between the basins was 

defined as a "point in the main stream of the Colorado 

River one mile below the mouth of the Paria River." Thus, 

the Lees Ferry gaging station and another gaging station 

installed in November 1923 on the Paria River became the 

measurement points used to determine compliance with 

the terms of the Compact. Under the Compact, the United 

States-Mexico Water Treaty of 1944, and the Colorado 

River Basin Project Act of 1968, 8.25 million acre-feet of 

water must pass to the Lower Basin each year, of which 

8.23 million acre-feet must pass the Lees Ferry gaging 

station (June 8, 1970, Criteria for coordinated long-range 

operation of Colorado River reservoirs pursuant to the 

Colorado River Basin Project Act of September 30, 1968). 
Introduction 3

Salt

Lake 

City

Las Vegas

Denver

Cheyenne

Phoenix

Los Angeles

San Diego

NEW MEXICO

UTAH

COLORADO

WYOMING

NEVADA

CALIFORNIA

MEXICO PACIFIC OCEAN

 GULF OF

CALIFORNIA

RIVER

River

RIVER

Green

R

Paria

R

Little Colorado R

Gila

River

ARIZONA

LEES FERRY

LAKE MEAD

RESERVOIR

TEXAS

09402500

LOWER BASIN

HOOVER DAM

IDAHO

09424000

09521000

09315000 09153000

09361500

09379500

GRAND CANYON 

NATIONAL PARK

USGS GAGING STATION

WITH STATION NUMBER

BOUNDARY OF

COLORADO RIVER 

DRAINAGE BASIN 

BOUNDARY BETWEEN 

THE UPPER AND LOWER 

BASINS

EXPLANATION

40

35

110 105

0 100 200

0 100 200

MILES

KILOMETERS

COLORADO

 GLEN 

CANYON 

 DAM

COLORADO

09363000

A.

RIVER-MILE 233.7

Base modified from Figure 1 in Smith and others (1960).

09380000

UPPER BASIN

San Juan

Figure 1. The Colorado River drainage basin and Lees Ferry. (A) The Colorado River Basin showing the location of Lees Ferry, Glen Canyon Dam, 

and the Lees Ferry and other U.S. Geological Survey gaging stations (with station numbers) used in this study. See table 1 for the official and 

shortened names of these stations.
4 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

EDGE OF COLORADO RIVER AT DISCHARGE OF 8,000 CUBIC

FEET PER SECOND 

LEES FERRY GAGE

LOWER STAFF GAGE

HIGH-ELEVATION TERRACES

LEES FERRY REACH AREA DEPICTED IN FIGURE 1C

NORTHING, IN FEET

EASTING, IN FEET

AREA OF GRAVEL BAR AT THE MOUTH OF THE PARIA RIVER 

HISTORICALLY INUNDATED AT HIGHER COLORADO RIVER DISCHARGES 

MODERN ROADS

PARIA RIVER

RIFFLE AT 

LONELY DELL

RANCH

EXPLANATION

GRAND CANYON NATIONAL PARK

GLEN CANYON

CATHEDRAL

WASH

LOW DISCHARGE

PARIA RIFFLE

(LOW DISCHARGE ONLY)

Base modified from orthorectified March 2000 aerial photography,

USGS Grand Canyon Monitoring and Research Center, Flagstaff, 

Arizona. Projection is 1983 Arizona State Plane, central zone.

MARBLE CANYON 

0

0

1

1 KILOMETER

MILE

B.

786,000788,000790,000792,000794,000796,000798,000800,000

2,124,000

2,126,000

2,128,000

2,130,000

2,132,000

2,134,000

2,136,000

AXEHANDLE

ALCOVE

Figure 1—Continued. The Colorado River drainage basin and Lees Ferry. (B) The Lees Ferry area showing the Lees Ferry 

Gage, the Lower Staff Gage, the area of the gravel bar at the mouth of the Paria River inundated by the Colorado River at 

higher discharges, and the high-elevation terraces between the Lower Staff Gage and Cathedral Wash studied by 

O’Connor and others (1994). See photographs in figs. 5A–C.
Introduction 5

3,150-FOOT ELEVATION CONTOUR

EDGE OF COLORADO RIVER AT DISCHARGE 

OF 5,000 CUBIC FEET PER SECOND

PEAK 1921 FLOOD EDGE OF WATER 

PEAK 1884 FLOOD EDGE OF WATER

NUMBER 1 GAGE (inclined section)

NUMBER 1 GAGE (vertical section)

NUMBER 2 GAGE

NUMBER 3 GAGE and

NUMBER 4 GAGE (inclined section) 

NUMBER 4 GAGE (vertical section on rock)

LEES FERRY GAGE 

CABLE GAGE

NORTHING, IN FEET

EASTING, IN FEET

USGS HYDROGRAPHER'S 

HOUSE, OFFICE, AND SEDIMENT LAB

UPPER CABLEWAY

LOWER CABLEWAY

MODERN CABLEWAY

FERRY 

CROSSING

DOWNSTREAM

HISTORICAL STAFF GAGES

MODERN PAVED ROAD, 

PARKING LOT, 

AND BOAT RAMP

EXPLANATION

DUGWAY GAGE

Base modified from 1:2,400 scale topographic maps, 

Bureau of Reclamation (1990). Projection is 1983 Arizona State 

Plane, central zone.

0

0

500 FEET

200 METERS

C.

795,500796,000796,500797,000797,500798,000798,500799,000799,500

2,132,000

2,132,500

2,133,000

2,133,500

2,134,000

2,134,500

2,135,000

2,135,500

Figure 1—Continued. The Colorado River drainage basin and Lees Ferry. (C) The Lees Ferry reach showing the Lees Ferry Gage, the 

six historical staff gages (Number 1 Gage, Number 2 Gage, Number 3 Gage, Number 4 Gage, Dugway Gage, and Cable Gage), and the 

extent of inundation at 5,000 cubic feet per second and during the 1921 and 1884 floods. Extent of inundation during the peak of the 

June 1921 flood is based on the peak stage of this flood at the Number 1, Dugway, and Number 4 staff gages and topography 

surveyed in 1990. Extent of inundation during the peak of the July 1884 flood is based on the assumption that the water surface was 

essentially flat upstream from the Lees Ferry Gage (as justified based on fig. 4A). 
6 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

Thus, measurements at Lees Ferry are essential to the 

management of water in the entire Colorado River 

drainage basin. These legal documents that govern the 

discharge of the Colorado River at Lees Ferry constitute 

part of the "The Law of the River" and are available 

electronically from the Bureau of Reclamation (Bureau of 

Reclamation, 2000). 

Widespread development of water resources within 

the Upper Colorado River Basin began with authorization 

of the Colorado River Storage Project in 1956 (Martin, 

1989). Glen Canyon Dam was the largest of the dams 

constructed under this act. The gates of this dam were 

closed on March 13, 1963, regulating the discharge of the 

Colorado River at Lees Ferry and initiating the storage 

of water in the reservoir known as Lake Powell. This 

reservoir filled for the first time on June 22, 1980, and its 

27 million acre-foot capacity is the second largest in the 

United States. The largest reservoir in the United States 

is Lake Mead, at the downstream end of Grand Canyon 

(fig. 1A).

Detailed analysis of the Lees Ferry discharge 

record is valuable for a variety of purposes, including 

evaluation of natural hydrologic variability within the 

Upper Basin and evaluation of the effects of upstream 

dams and diversions on the discharge of the Colorado 

River. Although water was exported from the headwaters 

of the Colorado River Basin as early as 1892 (Fradkin, 

1984), discharge at Lees Ferry was only slightly affected 

by upstream water development prior to the closure of 

Glen Canyon Dam in March 1963. Pre-1963 depletion of 

water from the headwaters of the Colorado River resulted 

in only a 10–15 percent reduction in the "virgin flow" of 

the Colorado River at Lees Ferry, with very little change 

in the amount of water depleted from the Upper Basin 

occurring between 1921 and 1963 (see fig. 4 in Ferrari, 

1988). Thus, analysis of the first 42 years of record 

at the Lees Ferry gaging station (from May 1921 to 

March 1963) provides information on the quasi-natural 

hydrology of the Upper Basin. The following 37 years 

of record at the gaging station (from March 1963 to 

September 2000) were dominated by the effects of 

upstream water development and regulation of discharge 

by operation of Glen Canyon Dam. Analysis of this 

later part of the period of record, therefore, provides 

information on the effect of upstream water development 

on the discharge of the Colorado River in Grand Canyon 

National Park, because no large tributaries enter the 

Colorado River between Glen Canyon Dam and the 

Lees Ferry gaging station.

Interest in the Lees Ferry discharge record 

increased after 1983 when comprehensive environmental 

studies were initiated by the Bureau of Reclamation to 

evaluate the effect of the operation of Glen Canyon Dam 

on the Colorado River ecosystem downstream from the 

dam (U.S. Department of the Interior, 1995). Previous 

analysis of the hydrologic effects of dam operation 

involved visual comparison of the record of stage and 

discharge for some years (Turner and Karpiscak, 1980), 

and visual and statistical comparison of post-dam 

bi-hourly, hourly, or 30-minute discharge data with predam daily mean discharge data (U.S. Department of the 

Interior, 1995). These analyses were hindered by the fact 

that only daily mean discharge data at the Lees Ferry 

gaging station were available prior to February 2, 1967, 

and 30-minute discharge data were only available after 

October 1, 1986. Thus, the value of these analyses was 

limited because pre-dam and post-dam hydrologic data at 

the Lees Ferry gaging station were not of comparable 

precision.

Table 1. U.S. Geological Survey gaging stations used in this study

Station number Official station name Shortened name used 

in this paper

09153000 Colorado River near Fruita, 

Colorado

Colorado River near 

Fruita

09315000 Green River at Green River, 

Utah

Green River 

09361500 Animas River at Durango, 

Colorado

Animas River

09363000 Florida River near Durango, 

Colorado

Florida River

09379500 San Juan River near Bluff, Utah near Bluff

09380000 Colorado River at Lees Ferry, 

Arizona

Lees Ferry

09402500 Colorado River near Grand 

Canyon, Arizona

Grand Canyon

09424000 Colorado River near Topock, 

Arizona

Topock

09521000 Colorado River at Yuma, 

Arizona

Yuma
Physical Characteristics of the Lees Ferry Gaging Reach 7

Purpose and Scope

This paper describes the results of a study that had 

three main goals. The first goal was to reanalyze, and 

revise if necessary, the record of floods for the period 

prior to construction of Glen Canyon Dam. The second 

goal was to rectify the differing precision between the 

various parts of the Lees Ferry gaging-station record 

by developing a continuous record of instantaneous 

discharge for the entire period of gage record (May 8, 

1921, through September 30, 2000). This computed 

record of instantaneous discharge is perhaps the longest 

(almost 80 years) high-resolution (mostly 15- to 30-

minute precision) times series of river discharge available 

for any river in the world. As used in this paper, the term 

"instantaneous discharge" is defined as the discharge of 

the river at any given instance in time. The third goal was 

to fully describe the hydrologic characteristics of the 

Colorado River at Lees Ferry during the pre- and postdam eras, and the implications for sediment transport and 

storage in the reach downstream from Lees Ferry in Grand 

Canyon National Park, through analyses of flow duration, 

sub-daily discharge variability, and flood frequency. These 

analyses were conducted on the Lees Ferry instantaneous 

discharge data for May 8, 1921, through September 30, 

2000, the available pre-gage historical flood data, and the 

4,500-year paleoflood data of O’Connor and others 

(1994).

It is anticipated that the basic data presented in this 

paper will be of value for a wide range of subsequent 

analyses by scientists and engineers. Most of the 

information presented in this paper is from unpublished 

USGS reports and gaging-station technical files stored in 

the Federal Records Centers, and was not previously 

accessible to the public. A complete list of all USGS files 

used in this paper and where they are stored is appended 

(Appendix A). The previously unavailable stage and 

discharge data presented in this paper can be either 

requested from the USGS Grand Canyon Monitoring and 

Research Center, Flagstaff, Arizona, or obtained 

electronically at http://www.gcmrc.gov. 

Acknowledgments

Funding for this study was provided by the Grand 

Canyon Monitoring and Research Center through a 

cooperative agreement with Utah State University and the 

National Research Program of the USGS. Lisa Dierauf 

did much of the early work on digitizing the first 10 years 

of the strip-chart record from the Lees Ferry Gage. 

Thoughtful reviews of earlier versions of this paper were 

provided by James Bennett, Randolph Parker, Theodore 

Melis, David Jay, John Gray, and James O’Connor. 

PHYSICAL CHARACTERISTICS OF THE

LEES FERRY GAGING REACH

The Lees Ferry gaging station on the Colorado 

River is located at the downstream end of Glen Canyon, 

and just upstream from Marble Canyon. Marble Canyon 

extends from Lees Ferry to the mouth of the Little 

Colorado River, and Grand Canyon extends from the 

mouth of the Little Colorado River to the Grand Wash 

Cliffs above Lake Mead reservoir (fig.1A). On the basis 

of the convention of Topping and others (2000), that 

portion of Grand Canyon between the mouth of the Little 

Colorado River and the Grand Canyon gaging station 

in fig. 1A is referred to as upper Grand Canyon. Today, 

there are only a few abandoned stone buildings and some 

recreational boating facilities at Lees Ferry. Between 1872 

and 1929, however, this ferry crossing was the only point 

within hundreds of miles where wagons or vehicles 

could cross the Colorado River, or where boats could be 

launched (Rusho and Crampton, 1992; Reilly, 1999). 

A nearby farm, the Lonely Dell Ranch (fig. 1B), was 

occupied from 1871 until 1974, and buildings at Lees 

Ferry, including the house and laboratory of the USGS 

hydrographer (fig. 1C), were occupied until 1976 (Reilly, 

1999).

The physiography of the Lees Ferry area is 

determined by the regional structure of the Echo Cliffs 

monocline, which crosses the Colorado River nearby. The 

upstream dip of the monocline brings the highly erodible 

Moenkopi and Chinle Formations to river level here 

(Hereford and others, 2000), and the Colorado River can 

be easily reached by following bedding planes within 

these formations to the river’s edge. In addition, Lees 

Ferry is also made accessible, because the Paria River has 

eroded an open valley in the Moenkopi Formation 1 mile 

downstream from the old ferry crossing on the Colorado 

River.
8 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

Stage and Discharge Measurements in the 

Lees Ferry Reach

The modern Lees Ferry Gage is located on the left 

(east) bank (figs. 1B–C), just upstream from the gravel bar 

at the mouth of the Paria River. It is housed in a concrete 

structure adjacent to the old dugway road to the ferry 

crossing, near the base of a cliff within the Shinarump 

Conglomerate member of the Chinle Formation (fig. 2). 

Since December 1966, all discharge measurements have 

been made from the Modern Cableway located 50 ft 

upstream from the Lees Ferry Gage (fig.1C). Prior to this 

date, discharge measurements were made from two 

cableways farther upstream. Most measurements were 

made from the cableway located 1 mile upstream and 

informally called the Upper Cableway (figs. 1C, 3A–B). 

At discharges exceeding about 60,000 ft3/s, discharge 

measurements were typically made from a second 

cableway located 0.4 miles upstream from the gage house. 

This second cableway crossed the river at a much wider 

cross-section and was informally called the Lower 

Cableway (figs. 1C and 3C–D).

The recording Lees Ferry Gage became operational 

on January 19, 1923. Prior to this date, stage was read 

twice daily on various staff gages in the 1-mile-long reach 

upstream (fig. 1C). These historical staff gages were 

known as the LaRue Gage, the Number 1 Gage, the 

Number 2 Gage, the Number 3 Gage, the Number 4 Gage, 

and the Dugway Gage. Each of these staff gages had a 

different period of record and datum. The LaRue Gage 

was installed on May 8, 1921, and was destroyed during 

the peak of the June 1921 flood; the Number 1 Gage was 

installed at the site of the LaRue Gage on June 24, 1921, 

but at a different datum; the Number 4 Gage was installed 

upstream on the left bank on August 3, 1921; and the 

Dugway Gage was installed on August 5, 1921, near the 

future location and at the same datum as the Lees Ferry 

Gage. The Number 2 and 3 Gages were used only during 

June–August 1921. Another staff gage, known as the 

Cable Gage, was installed on the left bank under the 

Upper Cableway in April 1924. Stage was read on this 

gage during most of the Upper Cableway discharge 

measurements until 1964. In March 1941, a final staff 

gage, herein referred to as the "Lower Staff Gage," was 

installed on the right bank of the Colorado River 1.5 miles 

downstream from the Lees Ferry Gage, below the mouth 

of the Paria River (fig. 1B). Stage was read on the Lower 

Staff Gage periodically until 1959. The important events 

in the history of the Lees Ferry gaging station are listed in 

Appendix B.

The Hydraulic Control and its Effect on the

Shape of the Stage-Discharge Rating Curve

The location and geometry of the hydraulic control 

for the Lees Ferry Gage changes as a function of stage. 

During large floods, these changes cause the water-surface 

profile in the reach downstream from the gage to flatten 

with increasing stage (fig. 4A). During the largest floods, 

this flattening of the water-surface profile (that is, the 

development of backwatered flow conditions) extends 

from some unknown point downstream in an upstream 

direction past the Lees Ferry Gage. This phenomenon 

produces a reversal in the curvature of the stage-discharge 

rating curve at a stage of about 15 ft (figs. 4B–C); above 

this stage, stage increases at a progressively faster rate 

than does discharge. This reversal in the curvature of the 

stage-discharge rating curve has not always been 

explicitly considered in estimations of the peak discharges 

of large floods nor in estimations of paleoflood discharges 

(O’Connor and others, 1994).

The existence of backwatered flow conditions near 

Lees Ferry at high stage is evident not only in the stage 

measurements at the staff gages, but also in photographs 

of the reach at higher discharge (figs. 5 and 6). At stages 

lower than about 10 ft (that is, discharges less than about 

20,000 ft3/s), the gravel bar at the mouth of the Paria River 

confines the flow to a channel adjacent to the left bank, 

and the constriction at the entrance to this channel forms a 

stable hydraulic control for the Lees Ferry Gage (fig. 5B). 

Under these conditions, a riffle exists in the left-bank 

channel, and a second riffle, called the Paria Riffle by 

modern river runners (Stevens, 1983), exists at a debris 

fan downstream from the mouth of the Paria River 

(figs. 1B, 4A, and 5B). As the stage increases above 10 ft, 

these riffles begin to "wash out" as the overall watersurface slope flattens (figs. 6A–B). At stages between 13 

and 15 ft (that is, discharges between about 40,000 and 

60,000 ft3/s), the flow divides into two or more channels 

across the gravel bar (fig. 5A), and the entire gravel bar 

forms the hydraulic control for the gage. Due to this 

change in the location and geometry of the hydraulic 

control, the water surface flattens considerably in the 0.3-

mile-long reach immediately downstream from the gage 

(figs. 4A and 6B); and at a stage of about 15 ft, the 

curvature of the stage-discharge rating curve reverses 

(fig. 4C). In contrast to the behavior of the water-surface 

slope in this reach, the water-surface slope in the reach 

upstream from the gage generally increases with 

increasing stage, and begins to flatten only at stages 

between 20 and 25 ft (figs. 4A, 6C–E). 
Physical Characteristics of the Lees Ferry Gaging Reach 9

A.

B.

Figure 2. Lees Ferry Gage. (A) View from the right bank 

downstream from the Number 1 Gage. Photograph taken by G.C. 

Stevens of the U.S. Geological Survey at 9:00 a.m. on September 

21, 1923, when the stage was 11.05 feet and the discharge was 

27,000 cubic feet per second. Source of this and the photographs 

in figs. 2B–3D, and 16B: 1921–37 Surface Water Records File, 

Colorado River at Lees Ferry, Arizona, file stored at the Federal 

Records Center in Laguna Niguel, California, in Accession No. 57-

78-0006, Box 2 of 2 , Location No. MB053635. (B) View from the 

edge of the dugway road downstream. Photograph taken by G.C. 

Stevens of the U.S. Geological Survey just after sunset on 

September 22, 1923.
10 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

A.

B.

Figure 3. Upper and Lower Cableways. (A) Upstream view of Upper Cableway and cable car. High-water marks from the June 1921 flood are visible 

on the right bank (on the left side of the photograph). Photograph taken by R.C. Rice of the U.S. Geological Survey on October 2, 1921. (B) Left bank Aframe and landing tower for the Upper Cableway. U.S. Geological Survey photograph taken from cable car over mid-channel in June 1939. The Cable 

Gage, installed in April 1924, is visible in the foreground below the left (upstream) support for the landing tower. 
Physical Characteristics of the Lees Ferry Gaging Reach 11

C.

D.

Figure 3—Continued. Upper and Lower Cableways.(C) Downstream view of right-bank A-frame and landing tower for the Lower 

Cableway. Lees Ferry Gage is in the distance on the left bank. U.S. Geological Survey photograph taken in June 1939. (D) View 

from right to left bank of Lower Cableway. Photograph taken by J.A. Baumgartner of the U.S. Geological Survey in June 1933.
12 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

A.

3,090

3,095

3,100

3,105

3,110

3,115

3,120

3,125

3,130

3,135

3,140

3,145

-15

-10

-5

0

5

10

15

20

25

30

35

-1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0

AT 5,000 CUBIC FEET PER SECOND

AT 60,000 CUBIC FEET PER SECOND

AT 90,000 CUBIC FEET PER SECOND

AT 120,000 CUBIC FEET PER SECOND

AT PEAK OF 1921 FLOOD

WATER-SURFACE ELEVATION AT PEAK OF 1884 FLOOD

TERRACE ELEVATIONS DOWNSTREAM FROM LOWER STAFF GAGE

ELEVATIONS OF "G1" DEPOSIT

ELEVATION ABOVE SEA LEVEL, IN FEET

STAGE AT LEES FERRY GAGE, IN FEET

STREAMWISE DISTANCE FROM LEES FERRY GAGE, IN MILES LOWER STAFF GAGE

RIFFLE AT

GRAVEL BAR

LONELY DELL 

RANCH

LEES FERRY GAGE

NUMBER 1 GAGE

NUMBER 4 GAGE

CABLE GAGE

BEST-FIT LINEAR REGRESSION CATHEDRAL WASH

PARIA

RIFFLE

2 POINTS FROM ANALYSIS OF 

1921–24 PHOTOGRAPHS

AXEHANDLE ALCOVE

EXPLANATION

WATER-SURFACE PROFILES

Figure 4. Flattening water-surface profiles with increasing stage and the reversal in curvature of the stagedischarge rating curve. (A) Water-surface profiles in the Lees Ferry reach at 5,000 cubic feet per second, 60,000, 

90,000, 120,000 cubic feet per second, and during the peak of the 1921 flood. Shown are locations of the gages 

and the Lonely Dell Ranch where stages were measured. Stages at the Cable and Number 4 Gages during the 

1921 flood were determined by analysis of the September–October 1921 and May 1924 photographs in 

Appendix B. Points between the gages on the 5,000 cubic feet per second profile were obtained from 1:2,400 

scale topographic maps (Bureau of Reclamation, 1990). Elevations of terraces capped by the G1 deposit of 

O’Connor and others (1994) in the reach below the Lower Staff Gage were determined using airborne LIDAR 

data collected by the Grand Canyon Monitoring and Research Center in March 2000. 

The reversal in the curvature of the stage-discharge 

rating curve was first recognized by the USGS in 1923, 

but was not accounted for in estimates of peak flows until 

1927. Because the USGS did not account for this reversal 

in curvature, slightly more water was computed passing 

the Lees Ferry Gage during high-discharge months than 

was computed passing the Grand Canyon gaging station 

prior to 1927. This problem motivated J.S. Gatewood and 

H.S. Hunter in 1938 (in an unpublished USGS report) to 

recompute the discharges during water years 1921 

through 1926 by fitting a mean stage-discharge rating 

curve to the data from this period. They then used the now 

standard shifting-control method (Kennedy, 1984, p. 25) 

to bring their discharges computed on the basis of this 

mean curve into better agreement with the measured 

discharges. The shifting-control method had not been 
Physical Characteristics of the Lees Ferry Gaging Reach 13

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

110,000

120,000

130,000

140,000

0 5 10 15 20 25

1921– 62 DISCHARGE MEASUREMENTS 

DISCHARGE, IN CUBIC FEET PER SECOND

STAGE, IN FEET

SMOOTHED CURVE FIT TO MEASUREMENTS

B.

0 5 10 15 20 25

SMOOTHED CURVE FIT TO 1921– 62 DATA 

SMOOTHED CURVE FIT TO WATER-YEAR 

1921– 22 DATA ONLY 

REVERSAL IN CURVATURE

C.

Figure 4—Continued. Flattening water-surface profiles with increasing stage and the reversal in curvature of the stage-discharge rating curve. 

(B) Stage-discharge rating curve at the Lees Ferry Gage defined by 4,114 discharge measurements made between August 3, 1921, and December 30, 

1962. Excluded from this figure are: (1) the data after 1962 because of the influence of Glen Canyon Dam, (2) the 29 pre-dam discharge measurements 

affected by ice, and (3) the 1 discharge measurement made on August 2, 1929, while backwatered flow conditions existed at the gage as a result of 

a large flood on the Paria River. (C) Smoothed curve fit to the data in part B (without the data plotted) showing the reversal in curvature that occurs 

at a stage of about 15 feet. As shown in part A, this reversal in curvature arises as the water-surface profile flattens in the 0.3-mile-long reach 

immediately downstream from the gage as the hydraulic control shifts downstream. Also plotted is the smoothed curve fit to only the 193 non iceaffected measurements made during the first 14 months of gage operation during water years 1921 and 1922. Although the stage-discharge rating 

curve varied from year to year, this graph illustrates that the mean 1921–62 and the 1921–22 rating curves were nearly identical. 

used to compute daily mean discharges at Lees Ferry prior 

to May 7, 1926, so the revision greatly improved the 

accuracy of the 1921–26 daily mean discharge data. 

Gatewood and Hunter used this approach to recompute 

the daily mean discharge for the following days: June 26, 

1921, through September 10, 1921; April 22, 1922, 

through July 17, 1922; April 17, 1923, through July 31, 

1923; April 8, 1924, through July 16, 1924; April 1, 1925, 

through July 21, 1925; and, April 16, 1926, through July 

20, 1926. On the basis of their computations, only the data 

with the largest errors, that is, the data from water years 

1921 through 1923, were revised and republished by the 

USGS in Grover and others (1939). 

At stages higher than about 19 to 20 ft (that is, 

discharges higher than about 110,000 to 120,000 ft3/s), 

the gravel bar at the mouth of the Paria River is almost 

overtopped (fig. 5C), and the progressive flattening 

of the water surface propagates upstream past the Lees 

Ferry Gage (fig. 6). At stages above 20 ft, the watersurface slope in the reach upstream from the gage is 

approximately equal to the water-surface slope in the 

reach downstream from the gage (fig. 4). A. Wilson 

(unpublished USGS analysis, April 20, 1962) compared 

the stage-discharge rating curves of the Lees Ferry 

Gage and the Lower Staff Gage and concluded that the 

hydraulic control for both gages was the same at stages 

above about 25 ft. Therefore, the decrease in the watersurface slope in the reach above the Lees Ferry Gage at 

stages between 20 and 25 ft is caused by the progressive 

development of backwatered flow conditions in the reach 

downstream from the gage, as the hydraulic control for 

the gage shifts farther downstream. 

Although its precise location is not known, the 

likely hydraulic control for the Lees Ferry Gage at stages 

greatly in excess of 20 ft is either at Badger Rapids, 

7.8 miles downstream from the gage, or at the riffle at the 

mouth of Cathedral Wash, 2.7 miles downstream from the 

gage (fig. 1B). In 1983, the water surface between the 

Lees Ferry Gage and Badger Rapids was observed to be 

relatively smooth at a discharge of about 90,000 ft3/s, with 

the riffle at Cathedral Wash "washed out" (Kenton Grua, 

river guide, oral commun., 2002); this observation 
14 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

A.

B.

Figure 5. Flow over the gravel bar at the mouth of the Paria River at a range of discharges. (A) Downstream view in June 1915. The Colorado 

River is flowing through the left-bank main channel and through two channels across the bar into the lower portion of the Paria River. 

Estimated discharge of the Colorado River is 50,000 cubic feet per second, based on records from the Yuma gaging station. The location of the 

Lonely Dell Ranch is indicated, where elevations of the high-water marks of the 1884 and 1921 floods were measured on June 7, 1922. Also 

shown are the future locations of the road and levee in part B, built by the National Park Service after regulation of flows by Glen Canyon Dam. 

Source of photograph: H.E. Gregory photographs 297 and 298, U.S. Geological Survey Photographic Library, Denver, Colorado. (B) Same view 

as in A on April 15, 1995. Discharge of the Colorado River is 9,500 cubic feet per second and flow is restricted to only the left-bank main 

channel. Photograph taken by D.J. Topping of the U.S. Geological Survey. 

suggests that the first hydraulic control downstream from 

Lees Ferry at high stage is Badger Rapids. Evidence for 

the Cathedral Wash riffle being the high-stage hydraulic 

control is provided by the presence of high-elevation 

terraces that extend downstream from the Lower Staff 

Gage (figs. 1B and 4A). The first high-elevation terraces 

occur on the right bank for about 700 feet downstream 

from the Lower Staff Gage, then continuously on the left 

bank for more than 0.6 miles, and terminate just above the 

riffle at Cathedral Wash. Although high-elevation terraces 

are present locally downstream, they are far less 

continuous than those above Cathedral Wash. Flood 

deposits preserved within the high-elevation terraces 

between the Lower Staff Gage and Cathedral Wash were 

first studied by O’Connor and others (1994). They 

determined that the uppermost unit within these terraces, 

the "G1" deposit, was likely deposited during a historical 

flood, which they speculated was the 1884 flood. In 

making this stratigraphic call, however, O’Connor and 

others (1994) did not have access to the historical data 

from the Lower Staff Gage. Extrapolation of the measured 

water-surface profile during the peak of the 1921 flood by 

means of a best-fit linear regression (fig. 4A) indicates that 

the G1 deposit was more likely formed during the 1921 

flood than during the 1884 flood. 
Physical Characteristics of the Lees Ferry Gaging Reach 15

C.

Figure 5—Continued. Flow over the gravel bar at the mouth of the Paria River at a range of discharges. (C) Upstream view on about June 1, 

1928. Discharge of the Colorado River is approximately 110,000 cubic feet per second. Bar is almost completely overtopped by the Colorado 

River, and backwatered conditions extend far up the Paria River. At lower discharges, a riffle is present in the left-bank channel (in foreground). 

At 110,000 cubic feet per second, however, the riffle is washed out as the hydraulic control for the Lees Ferry Gage shifts farther downstream. 

Photograph taken by the U.S. Geological Survey. Source of this photograph: wall display in the Flagstaff, Arizona U.S. Geological Survey office. 

Pre-Dam Hysteresis in the Water-Surface 

Slope in the Reach Upstream from the 

Lees Ferry Gage

In a one-dimensional sense, the longitudinal watersurface slope in a river at a given sub-critical discharge is 

governed by the geometry of the downstream hydraulic 

control, cross-sectional geometry, streamwise changes 

in cross-sectional geometry, and bed roughness (for 

example, see Chow, 1959). Thus, the longitudinal watersurface slope can vary as a function of the volume of 

sediment stored in pools because changes in the volume 

of stored sediment can affect the mean cross-sectional 

geometry, streamwise variation in cross-sectional 

geometry, and bed roughness. Analysis of the watersurface-slope data in the reach upstream from the Lees 

Ferry Gage indicates that prior to the construction of 

Glen Canyon Dam, seasonal scour and fill of the pools 

upstream from the gage (Leopold and Maddock, 1953; 

Colby, 1964; Howard and Dolan, 1981; Burkham, 1986; 

Topping and others, 2000; Grams and others, Utah State 

University, written commun., 2002) caused hysteresis in 

the water-surface profile, with the water-surface slope 

being steeper during the rising limb than during the 

receding limb of the annual snowmelt flood (figs. 6C–E). 

Following the extensive scour of these pools during high 

dam releases in 1965 (Pemberton, 1976; Williams and 

Wolman, 1984; Burkham, 1986; Grams and others, Utah 

State University, written commun., 2002), however, this 

secondary control on water-surface slope near Lees Ferry 

ceased to be important.

The effect of the seasonal pre-dam scour and fill 

of the pools upstream from the Lees Ferry Gage on the 

water-surface slope is evident in figs. 6C–E. As shown 

in fig. 6B of Topping and others (2000), the bed at the 

Upper Cableway scoured each spring during the rising 

limb of the annual snowmelt flood, then filled during the 

receding limb (with only a slight lag between the time of 

the flood peak and the time of maximum scour). Colby 

(1964) and Topping and others (2000) concluded that 

scour and fill of the bed at the Upper Cableway was 

largely driven by the interaction between the flow and the 

geometry of the reach, and not by changes in the upstream 

sediment supply [as was the case at the Grand Canyon 

gaging station downstream (Topping and others, 2000)]. 
16 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

-0.0005

0.0000

0.0005

0.0010

0.0015

5 10 15 20 25 30

WATER-SURFACE SLOPE 

LEES FERRY GAGE TO LONELY DELL RANCH

1921 FLOOD

B.

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

5 10 15 20 25 30

WATER-SURFACE SLOPE 

LEES FERRY GAGE TO LOWER STAFF GAGE

1941 FLOOD

A.

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

5 10 15 20 25 30

NOVEMBER 1 THROUGH FLOOD PEAK 

FLOOD PEAK THROUGH OCTOBER 31 

WATER-SURFACE SLOPE 

NUMBER 1 GAGE TO LEES FERRY GAGE

1921 FLOOD

C.

STAGE AT LEES FERRY GAGE, IN FEET STAGE AT LEES FERRY GAGE, IN FEET

Figure 6. Water-surface slopes as a function of stage: (A) In the 1.5-mile-long reach between the Lees Ferry Gage and the Lower Staff Gage, 

determined from 27 stage measurements made at the 2 gages during 1941 and 1942, and 11 stage measurements made at the 2 gages during 1958 

and 1959. (B) In the 0.3-mile-long reach between the Lees Ferry Gage and the lower reach of the Paria River at the Lonely Dell Ranch, determined from 

the June 7, 1922, surveys of the June 1921 high-water marks, and from 30 surveys made during higher-water periods in 1927, 1928, and 1929. Because 

the Lees Ferry Gage and the Lonely Dell Ranch are on opposite banks of the river above the gravel bar, there is a cross-stream component of the watersurface slope in this figure that would be absent if these sites were on the same side of the river. This leads to the water-surface slope between the 

gage and the ranch becoming negative at stages above 17 feet (at discharges above 90,000 cubic feet per second). At extremely high stages, as during 

the 1921 flood, the water-surface slope is of the order of 0.00001, as fully backwatered flow conditions develop in this reach. (C) In the 820-foot-long 

reach between the Number 1 Gage and the Lees Ferry Gage, determined from 136 stage measurements made at the two gages between June 18, 

1921, through September 15, 1922. Similar to the case in part B, because the Number 1 Gage and the Lees Ferry Gage are located on opposite banks of 

the river in a meander, there is a cross-stream component of the water-surface slope in this figure. Data in this figure and in parts D–E are segregated 

into: (1) slopes measured each year between November 1 and the peak of the subsequent snowmelt flood, and (2) slopes measured each year between 

the peak of the snowmelt flood and October 31. Because of the seasonal scour and fill of sediment in the pools upstream from the Lees Ferry Gage, 

hysteresis during the annual snowmelt flood (indicated by arrows) exists in the water-surface slope in this figure and in parts D–E. 
Colorado River Floods at Lees Ferry 17

0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

5 10 15 20 25 30

NOVEMBER 1 THROUGH FLOOD PEAK 

FLOOD PEAK THROUGH OCTOBER 31

WATER-SURFACE SLOPE 

NUMBER 4 GAGE TO LEES FERRY GAGE

STAGE AT LEES FERRY GAGE, IN FEET

1921 FLOOD

D.

0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

5 10 15 20 25 30

NOVEMBER 1 THROUGH FLOOD PEAK 

FLOOD PEAK THROUGH OCTOBER 31 

WATER-SURFACE SLOPE 

CABLE GAGE TO LEES FERRY GAGE

STAGE AT LEES FERRY GAGE, IN FEET

1921 FLOOD

E.

Figure 6—Continued. Water-surface slopes as a function of stage: (D) In the reach between the Number 4 Gage and the Lees Ferry Gage, 

determined from 209 stage measurements made at the 2 gages between August 14, 1921, and September 22, 1923. Peak stage during the 

June 1921 flood at the Number 4 Gage was determined by analysis of the photographs taken by R.C. Rice in September–October 1921 (figs. B1 and B3). 

(E) In the reach between the Cable Gage (490 feet upstream from the Number 4 Gage) and the Lees Ferry Gage, determined from 3,196 stage 

measurements made at the 2 gages between April 26, 1924, and July 30, 1962. Peak stage during the June 1921 flood at the Cable Gage was 

determined by analysis of the photographs taken by R.C. Rice in September–October 1921 and W.E. Dickinson in May 1924 (figs. B1 and B5).

The bed generally scoured between April and the third 

week in June each year, and filled between the third week 

in June and the end of October (Topping and others, 2000, 

fig. 6B). At stages less than about 15 ft (the stage of the 

reversal of curvature in the stage-discharge rating curve), 

the water-surface slope in the reach upstream from the 

gage was about a factor of two steeper during the months 

when the bed was scouring than it was during the months 

when the bed was filling. Above a stage of about 15 ft, the 

water-surface slopes during the rising and receding limbs 

of the annual snowmelt flood were approximately the 

same in the reach upstream from the gage. 

COLORADO RIVER FLOODS AT

LEES FERRY

It is important to determine accurately the 

magnitudes of the largest floods in the recent history of 

the Lees Ferry gaging station because these floods

may have had a large role in determining the present 

morphology of the Colorado River, and because 

these floods are an essential component of the natural 

hydrology of the pre-dam Colorado River. Although the 

magnitudes of floods after August 1921 were measured 

from existing cableways, the magnitudes of earlier 

floods can only be estimated from direct or indirect 

measurements of flood stage and estimates of the 

discharges at these stages. 

There is no way to determine the maximum stage 

of every annual flood before August 1921, but it is 

possible to estimate the discharges of the two largest 

floods during the 37 years prior to installation of the 

cableway because both floods were observed. A flood 

whose stage was higher than any in subsequent years 

occurred in June 1921, 1 month after the first staff gage 

was installed and 2 months prior to the installation of the 

cableway. An even higher-discharge flood in 1884 

was witnessed by Jerry Johnson, the ferry operator, and 

his family. The date of the peak of the 1884 flood at Lees 

Ferry has been reported to have been either June 18 

(Reilly, 1999) or July 7 (LaRue, 1925; Patterson and 

Somers, 1966). Because the peak of this flood was 

observed at the Yuma gaging station (660 miles 
18 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

downstream from Lees Ferry) on June 28, the correct date 

for the peak of the 1884 flood at Lees Ferry is probably 

June 18. Estimation of the peak magnitudes of the 1884 

and 1921 floods depends on the accurate determination of 

maximum flood stages and appropriate extrapolations of 

the stage-discharge rating curves at the various staff gages 

in the Lees Ferry reach. Because of the importance of the 

1884 and 1921 floods, all available information about the 

magnitudes of the peak stage and the methods by which 

discharge was estimated from the stage data for each of 

these floods was assembled and reviewed. 

Estimate of the Peak Discharge of the 

1921 Flood at Lees Ferry

Because estimation of the peak discharge of the 

1884 flood depends on estimation of the peak discharge 

of the lower June 1921 flood, estimation of the peak 

discharge of the 1921 flood is described first. The peak 

discharge of the 1921 flood at Lees Ferry was unknown 

and had to be estimated, but the peak stage was precisely 

measured at the Number 1 Gage, the Dugway Gage, 

and at two different locations at the Lonely Dell Ranch. 

Analysis of photographs taken of the Number 4 and Cable 

Gages in 1921–24 (Appendix B) allow the peak stage of 

the 1921 flood to be determined at these two additional 

sites, but with less precision (fig. 4A).

The original USGS estimate of the peak 

discharge of the June 1921 flood was 174,000 ft3/s; this 

value was based on extrapolation of the first available 

stage-discharge rating curve for the Number 1 Gage 

(unpublished USGS 1921 annual technical file for the 

Lees Ferry gaging station). This rating curve was based on 

120 discharge measurements made between August 5, 

1921, and June 15, 1922. The original estimate of 

174,000 ft3/s was not published. The first published 

estimate of the peak discharge of the 1921 flood at Lees 

Ferry was "about 190,000 ft3/s " (Grover and others, 

1923); this value was based on comparison of the 

estimated daily mean discharges during the 1921 flood 

at Lees Ferry with those measured downstream at the 

Topock and Yuma gaging stations. This value of "about 

190,000 ft3/s " was the official USGS estimate of the peak 

discharge of the June 1921 flood until 1939, when the 

USGS revised the estimate to 220,000 ft3/s (Grover and 

others, 1939), on the basis of the 1938 study of J.S. 

Gatewood and R.S. Hunter. The 1939 revision remains 

the currently accepted estimate of the peak discharge of 

this flood.

The Key Assumption of J.S. Gatewood and 

R.S. Hunter

The 1939 revision of the estimate of the peak 

discharge of the 1921 flood was based on a key 

assumption now known to be false. Because the reversal 

in the curvature of the stage-discharge rating curve at 

the Lees Ferry Gage made extrapolation of this curve 

difficult, Gatewood and Hunter decided to estimate the 

peak discharge of the 1921 flood at the Grand Canyon 

gaging station. They chose the Grand Canyon gaging 

station over Lees Ferry as the place to perform this 

extrapolation because (1) the stage-discharge rating curve 

was more stable at the Grand Canyon gaging station than 

at the Lees Ferry Gage, and (2) there was no known 

reversal in the curvature of the stage-discharge rating 

curve at the Grand Canyon gaging station. The chief 

problem with this approach, however, was that unlike at 

Lees Ferry, where the high-water marks from the 1921 

flood were at known stages, there was no certainty of 

the peak stage of the 1921 flood at the Grand Canyon 

gaging station because this station was not established 

until November 1922. Unfortunately, Gatewood and 

Hunter did not correctly interpret the dates of the various 

high-water marks at the Grand Canyon gaging station, 

and mistook the likely 1884 high-water mark for the 

1921 high-water mark. This misinterpretation led to an 

overestimation of the peak discharge of the 1921 flood 

by about 30 percent.

Gatewood and Hunter assumed that the highwater mark found in 1933 by J.A. Baumgartner of the 

USGS in a left-bank recess upstream from the 

measurement cableway at the Grand Canyon gaging 

station was from the 1921 flood. By extrapolation of the 

stage-discharge rating curve at the Grand Canyon gaging 

station to the stage of this high-water mark (37.5 ft), they 

estimated that the peak discharge of the 1921 flood was 

about 220,000 ft3/s at the Grand Canyon gaging station. 

Because very little tributary inflow or attenuation of the 

peak discharge occurred between the Lees Ferry and 

Grand Canyon gaging stations during snowmelt floods, 

they then assumed that the peak discharge of the 1921 

flood at Lees Ferry was also 220,000 ft3/s. 
Colorado River Floods at Lees Ferry 19

To make the stage-discharge rating curve at the 

Lees Ferry Gage pass through a discharge of 220,000 ft3/s 

at the measured peak stage of the 1921 flood (26.5 ft), 

Gatewood and Hunter imposed a second reversal in 

curvature at a stage of 17 ft (fig. 7). As shown in a 

preceding section of this paper, the water-surface slope at 

the Lees Ferry Gage decreases with increasing stage as a 

result of the progressive development of backwatered flow 

conditions in the reach. When water-surface profiles 

progressively flatten with increasing stage, discharge is 

proportional to stage raised to a power less than one. 

Beginning in water year 1939, the USGS adopted the 

1938 assumption of Gatewood and Hunter that there is a 

second reversal in the curvature of the Lees Ferry stagedischarge rating curve at a stage of about 17 ft. Thus, they 

assumed that, at discharges above about 85,000 ft3/s, 

discharge returned to being proportional to stage raised 

to a power greater than one. Dickinson (1944) reiterated 

this assumption, stating that a "moderate reversal in the 

station rating curve occurs in the range from 60,000 to 

80,000 second-feet [ft3/s], above and below which rating 

has normal curvature." Though this assumption was 

made, it was not supported by the data (fig. 8). Analysis 

of the data from the six highest flow years at Lees 

Ferry provide no support for the existence of this second 

reversal in curvature. No second reversal in curvature is 

evident in any of the individual years with substantial data 

above a stage of 17 ft (water years 1921–22, 1928, 1941, 

1949, 1952, and 1957, fig. 8).

An earlier USGS memorandum written by G.C. 

Stevens on May 25, 1925, suggests that the more likely 

peak stage of the 1921 flood at the Grand Canyon 

gaging station was 33 ft, not 37.5 ft. As stated in this 

memorandum, on September 13, 1923, J.W. Johnson, 

resident USGS hydrographer at the Grand Canyon gaging 

station, wrote that the highest visible high-water mark on 

the cliff on the south side of the river at the Grand Canyon 

gaging station was at a stage of about 32 ft. This highwater mark was at a much higher stage than the peak stage 

of any flood at this gaging station since its establishment 

in November 1922. On March 14, 1924, G.G. Sykes of the 

USGS used a level to determine that the stage of this highwater mark was 33 ft (figs. 9A–C). Because it was much 

higher than the peak stage of any flood on the Colorado 

River after June 1921, the high-water mark at a stage of 

33 ft was probably from the June 1921 flood.

These data from the early 1920s suggest that the 

high-water mark at a stage of 37.5 ft found in 1933 at the 

Grand Canyon gaging station was probably from the 1884 

flood, and not from the 1921 flood. The 1884 flood was 

the largest historical flood prior to 1921, and the peak 

stage of this flood was about 4 ft higher than that of the 

1921 flood at Lees Ferry. If the high-water mark at a stage 

of 37.5 ft was from the 1921 flood, then the high-water 

mark at a stage of 33 ft should have been erased by the 

1921 flood, and should not have been prominent in 

photographs taken at the site in November 1921 (figs. 9A–

B). Power-law extrapolation of the stage-discharge rating 

curve fit to the pre-dam data from the Grand Canyon 

gaging station suggests that, at a stage of 33 ft, the peak 

discharge of the 1921 flood would be about 163,000 ft3/s, 

and, at a stage of 37.5 ft, the peak discharge of the 1884 

flood would be about 214,000 ft3/s. Extrapolation along 

this same curve suggests that the peak discharge of a 

prehistoric flood associated with the highest high-water 

marks found near the gage by the National Park Service 

during construction of the Kaibab Bridge would be in 

excess of 300,000 ft3/s, and possibly as high as about 

360,000 ft3/s (fig. 9D). 

Other Measurements of the Peak Discharge of the 

1921 Flood Made Downstream and Upstream from Lees Ferry

Peak discharges measured at gaging stations 

downstream and upstream from Lees Ferry during the 

June 1921 flood indicate that the original USGS estimate 

of the peak discharge of the June 1921 flood at Lees Ferry 

of 174,000 ft3/s was probably correct, because these other 

measurements range between 167,000 and 188,000 ft3/s. 

None of the observations made at these other gaging 

stations indicate that the discharge was as high as the 

220,000 ft3/s value estimated by Gatewood and Hunter 

in 1938.

Comparison with measurements made at 

downstream gaging stations is valid because snowmelt 

floods on the tributaries entering the Colorado River 

downstream from Lees Ferry would have peaked 

earlier than late June. Thus, there would have been very 

little inflow from these tributaries to increase the peak 

discharge substantially. Furthermore, because the volume 

of water within the 1921 snowmelt flood was large and 

the duration of this flood was long, there would have been 

very little attenuation in the peak discharge of this flood 

between Lees Ferry and downstream gaging stations. 
20 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000

180,000

200,000

220,000

240,000

5 10 15 20 25 30 35

WATER-YEAR 1921–22 DATA AT LEES FERRY 

SMOOTHED CURVE FIT TO WATER-YEAR 1921–22 LEES FERRY DATA

LEES FERRY RATING CURVE OF GATEWOOD AND HUNTER 

ORIGINAL PUBLISHED PEAK DISCHARGE OF THE JUNE 1921 FLOOD 

AT THE TOPOCK GAGING STATION 

RANGE OF UNCERTAINTY IN THE PEAK DISCHARGE OF THE JUNE 1921 

FLOOD AT THE YUMA GAGING STATION

PROBABLE COMBINED DISCHARGE OF THE UPPER COLORADO, 

GREEN, AND SAN JUAN RIVERS CONTRIBUTING TO THE PEAK DISCHARGE 

OF THE JUNE 1921 FLOOD AT LEES FERRY 

DISCHARGE, IN CUBIC FEET PER SECOND

STAGE MEASURED AT THE LEES FERRY GAGE, IN FEET

REVERSAL IN CURVATURE

SECOND REVERSAL IN CURVATURE

IMPOSED BY GATEWOOD AND HUNTER

EXPLANATION

Figure 7. Comparison of the stage-discharge rating curve defined by the discharge measurements made during water-years 1921–22 

with the revised stage-discharge rating curve of Gatewood and Hunter (unpublished U.S. Geological Survey report, 1938). Though their 

revised curve plotted slightly high relative to the measurements at stages of 18 to 20 feet, they corrected for this in their revision of the 

1922–26 data by using the shifting-control method. Also shown are: the original published peak June 22, 1921, discharge of the 

Colorado River at the Topock gaging station; the peak June 27, 1921, discharge (with uncertainty) of the Colorado River at the Yuma 

gaging station; and the probable combined discharge of the upper Colorado, Green, and San Juan Rivers contributing to the June 18, 

1921, peak at Lees Ferry. 
Colorado River Floods at Lees Ferry 21

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

110,000

120,000

130,000

140,000

6 8 10 12 14 16 18 20 22

WATER-YEAR 1921–22 DATA 

A.

SMOOTHED CURVE FIT TO DATA

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

110,000

120,000

130,000

140,000

6 8 10 12 14 16 18 20 22

WATER-YEAR 1928 DATA 

B.

SMOOTHED CURVE FIT TO DATA

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

110,000

120,000

130,000

140,000

6 8 10 12 14 16 18 20 22

WATER-YEAR 1941 DATA 

DISCHARGE, IN CUBIC FEET PER SECOND

C.

SMOOTHED CURVE FIT TO DATA

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

110,000

120,000

130,000

140,000

6 8 10 12 14 16 18 20 22

WATER-YEAR 1949 DATA 

D.

SMOOTHED CURVE FIT TO DATA

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

110,000

120,000

130,000

140,000

6 8 10 12 14 16 18 20 22

WATER-YEAR 1952 DATA 

STAGE, IN FEET

E.

SMOOTHED CURVE FIT TO DATA

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

110,000

120,000

130,000

140,000

6 8 10 12 14 16 18 20 22

WATER-YEAR 1957 DATA 

F.

SMOOTHED CURVE FIT TO DATA

Figure 8. Stage-discharge rating curves at the Lees Ferry Gage defined by discharge measurements during the 6 high-discharge water years with 

substantial data above a stage of 17 feet. (A) 1921–22. (B) 1928. (C) 1941. (D) 1949. (E) 1952. (F) 1957. 
22 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

A.

B.

Figure 9. High-water marks from the 1921 flood and estimated 

peak discharges of the 1921 and 1884 floods at the Grand 

Canyon gaging station. (A) November 16, 1921, upstream view of 

the future reach of the Grand Canyon gaging station. Visible on 

the left bank (right side of photograph) is the likely high-water 

mark from the 1921 flood. This high-water mark is at the same 

elevation as the highest high-water mark in parts B and C. "1" 

indicates the position of a reference point on the cliff common to 

this view and to the view in part B. The lower high-water mark 

was probably produced during a flood that had a peak discharge 

of 67,000 cubic feet per second at the Lees Ferry Gage on August 

25, 1921. Photograph taken by R.C. Rice of the U.S. Geological 

Survey. Source of this and the photographs in parts B and C: 

Gaging station reconnaissance and construction reports, 

Colorado River near Grand Canyon, Arizona, file stored at the 

Federal Records Center in Denver, Colorado, in Accession No. 57-

64A-0423, Box 1 of 24 , Location No. 62478. (B) November 16, 

1921, view of the cliff on the left (south) bank of the Colorado 

River across from the future site of the lower (right-bank) gage at 

the Grand Canyon gaging station. "1" indicates the position of the 

reference point in part A. Visible are the likely high-water marks 

from the June 1921 flood and the lower-magnitude August 1921 

flood. "2" and "3" indicate the positions of two reference points 

common to this view and to the view in part C. Measurements 

made with hand levels in December 2000 (N.J. Hornewer, U.S. 

Geological Survey, written commun., 2000) and July 2001 

confirm that the likely June 1921 high-water mark in this 

photograph and in part C was the high-water mark at a stage of 

33 feet measured by G.G. Sykes in 1924. Photograph taken by 

R.C. Rice of the U.S. Geological Survey. 
Colorado River Floods at Lees Ferry 23

0

50,000

100,000

150,000

200,000

250,000

300,000

350,000

400,000

0 10 20 30 40 50

1922–63 DATA 

POWER-LAW EXTRAPOLATION 

OF THE RATING CURVE 

DEFINED BY THE 1922–63 DATA

DISCHARGE, IN CUBIC FEET PER SECOND

STAGE AT THE LOWER (RIGHT-BANK) GAGE AT

THE GRAND CANYON GAGING STATION, IN FEET 1921 FLOOD 1884 FLOOD

LARGEST KNOWN FLOOD

C.

D.

Figure 9—Continued. High-water marks from the 1921 flood and estimated peak discharges of the 1921 and 1884 floods at the 

Grand Canyon gaging station. (C) Fall 1922 view of the cliff in part B. "2" and "3" indicate the positions of the two reference points in part 

B. The likely high-water mark from the June 1921 flood is still visible 1 year after the photograph in part B was taken. Also visible is the 

likely high-water mark from the June 1922 flood that had a peak discharge of 116,000 cubic feet per second at Lees Ferry. Note that the 

high-water mark from the 67,000 cubic feet per second August 1921 flood has been erased by the larger, more recent June 1922 flood. 

Photograph taken by F.C. Ebert of the U.S. Geological Survey during the construction of the Grand Canyon gaging station. (D) Stagedischarge rating curve at the lower gage at the Grand Canyon gaging station defined by 3,703 pre-dam discharge measurements made 

between November 12, 1922, and March 6, 1963. The "largest known flood" refers to the flood associated with the highest high-water 

mark found by the National Park Service on September 24, 1919, during the construction of the Kaibab (Black) Bridge (bridge shown in 

parts A–C). Description of the bridge construction was published in the Engineering News Record of February 10, 1921. 
24 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

Therefore, the peak discharge of the 1921 flood at the 

Lees Ferry should have been similar to that at the Topock 

gaging station, which is approximately 450 miles 

downstream from Lees Ferry, and the Yuma gaging 

station, which is approximately 660 miles downstream 

from Lees Ferry (fig. 1A). Support for this is provided by 

the measured peak discharges during the high-discharge, 

large-volume, long-duration snowmelt floods of 1922 and 

1923, which had virtually identical peak discharges at the 

Lees Ferry, Topock, and Yuma gaging stations (U.S. 

Geological Survey, daily mean and peak discharge data, 

accessed November 22, 2000). During the 1922 snowmelt 

flood, peak discharges at the Lees Ferry, Topock, and 

Yuma gaging stations were 116,000 ft3/s, 125,000 ft3/s 

and 117,000 ft3/s, respectively; during the 1923 snowmelt 

flood, peak discharges at the Lees Ferry, Topock, and 

Yuma gaging stations were 98,300 ft3/s, 103,000 ft3/s, and 

101,000 ft3/s, respectively. Only during lower volume 

snowmelt floods, with peaks of much shorter duration 

than during the 1921, 1922, or 1923 floods, was 

substantial attenuation of the peak discharge observed 

between the Lees Ferry and Yuma gaging stations.

The estimated peak discharge of the 1921 flood at 

the Topock gaging station was 174,000 ft3/s on June 22, 

1921 (Grover and others, 1922). This peak discharge was 

estimated by extrapolating the stage-discharge rating 

curve that applied to this site in 1921. This rating curve 

was defined by 30 discharge measurements covering a 

discharge range between 8,000 and 80,000 ft3/s (Grover 

and others, 1922). Based on an evaluation of the discharge 

data at the Topock gaging station by Dickinson (1944), the 

174,000 ft3/s estimate of the peak discharge may be, if 

anything, too high. Dickinson (1944) stated, "In general, 

records prior to 1924 tend to show the discharge too 

great above about 20,000 second-feet [ft3/s] owing to 

equipment and methods then in use in making discharge 

measurements." In 1942, the USGS revised its estimate of 

the 1921 flood at the Topock gaging station to "greater 

than 200,000 ft3/s" (Parker and others, 1942), but there 

was no basis for this revision except to make it consistent 

with Gatewood and Hunter’s 1938 revision of the 

estimated peak discharge of the 1921 flood at Lees Ferry.

Discharge data collected farther downstream at the 

Yuma gaging station during the 1921 flood are 

inconsistent with Gatewood and Hunter’s 1938 estimate 

of the peak discharge at Lees Ferry and Parker and others’ 

1942 revision of the peak discharge at the Topock gaging 

station. The peak discharge of the 1921 flood measured at 

the Yuma gaging station was 188,000 ft3/s on June 27, 

1921 (Grover and others, 1922). This value was never 

revised by the USGS and is still the accepted peak 

discharge at Yuma during the 1921 flood. The computed 

discharges at the Yuma gaging station during 1921 were 

based on 164 measurements made during the year (Grover 

and others, 1922). Indeed, a discharge measurement was 

made on the morning of June 27, 1927, near the peak of 

the flood (fig. 10).

The published estimate of 188,000 ft3/s at the Yuma 

gaging station may be slightly high because (1) the crosssectional area of the flow at the Yuma gaging station was 

determined by primitive sounding methods that would 

have overestimated the cross-sectional area of flow, and 

(2) the mean velocities in the higher flow discharge 

measurements during 1921 were estimated by multiplying 

surface velocities by a coefficient of 0.9 (Dickinson, 

1944). In evaluating the discharge data at the Yuma gaging 

station, Dickinson (1944), stated:

Prior to 1926, measurements are generally subject to 

errors of varying amounts due to methods and equipment 

used, including the use of relatively few measuring points. 

During 1911–15 and at stages above low-water during 

1916–22, most measurements were based on observations 

of surface velocity using a coefficient of 0.9 to obtain the 

mean. At stages above low-water, soundings were made 

separately from velocity observations prior to 1926, and 

prior to 1923 a cylindrical weight suspended from one end 

(axis vertical) was used which was apt to be carried 

downstream by drift and high velocity, resulting in too large 

soundings. Soundings and velocity observations were 

further complicated by a stayline 1918–25. 

When the roughness elements on the bed are 

small relative to the flow depth, velocity profiles in steady, 

uniform flow tend to have a logarithmic shape in only 

the lower 20 percent of the flow, and an approximately 

parabolic shape in the upper 80 percent of the flow 

(Rattray and Mitsuda, 1974; Wiberg and Smith, 1991; 

Long and others, 1993). The coefficient relating surface to 

mean velocity in this type of profile is not 0.9, but 0.8 (at 

one significant figure). The peak discharge of the 1921 

flood at the Yuma gaging station, based on a velocity 

coefficient of 0.8 and the assumption that the sounding 

methods did not introduce too much error, is 167,000 ft3/s. 

The peak discharge of this flood at the Yuma gaging 

station was therefore between 167,000 and 188,000 ft3/s; 

the uncertainty of this estimate is associated with the 

accuracy of the sounding and the cross-sectionally 

averaged shape of the velocity profile.
Colorado River Floods at Lees Ferry 25

Figure 10. Discharge measurement being made by the U.S. Reclamation Service (the former name of the Bureau of Reclamation) on the morning of June 

27, 1921, at the Yuma gaging station, near the peak of the June 1921 flood. At the time of the photograph, measured discharge (based on the measured 

surface velocity and a coefficient of 0.9) was 182,000 cubic feet per second, and stage was 31 feet. Photo "Yuma 1094" taken by the U.S. Bureau of 

Reclamation. Source of this photograph: 1891–1937 Surface Water Records File, Colorado River at Yuma, Laguna Niguel Federal Records Center, 

Accession No. 57-78-0006, Box 2 of 2, Location No. MB053635.

The June 18 peak discharge of the 1921 flood at 

Lees Ferry can also be estimated on the basis of the 

records from upstream gaging stations. Based on an 

extrapolation of the stage-discharge rating curve above 

40,000 ft3/s, the June 16, 1921, peak discharge at the 

Colorado River near Fruita gaging station (fig. 1A) was 

estimated to be 81,100 ft3/s (Grover and others, 1922). 

The June 17, 1921, peak discharge at the Green River

at Green River gaging station (fig. 1A) was computed 

to be 65,500 ft3/s (Grover and others, 1922). The stagedischarge rating curve for the Green River at this site was 

well defined by discharge measurements up to 70,000 ft3/s 

(Grover and others, 1922), so 65,500 ft3/s was probably 

an accurate value.

Although no gaging stations were operating 

on the mainstem of the San Juan River during 1921, 

gaging stations on two of the major tributaries, the 

Animas and Florida Rivers, were used to estimate the 

likely discharge contribution of the San Juan River to the 

June 18, 1921, peak discharge at Lees Ferry (fig. 1A). The 

first decade of overlap between the Animas and Florida 

River gaging stations with the near Bluff gaging station, 

the most downstream gaging station on the San Juan 

River, was 1928–37. During May through June in this 

period, the combined daily mean discharge of the Animas 

and the Florida Rivers accounted for about 40 percent of 

the next day’s discharge of the San Juan River at the 

near Bluff gaging station (fig. 11); thus, the daily mean 

discharge of the San Juan River at the near Bluff gaging 

station during May 1921 was estimated by multiplying 

the combined previous day’s discharge of the Animas 

and Florida Rivers by a factor of 2.5. On June 16, 1921, 

the combined daily mean discharge of the Animas and 

Florida Rivers was about 8,000 ft3/s. Thus, it is likely 

that the San Juan River contributed about 20,000 ft3/s 

to the June 18, 1921, peak discharge at Lees Ferry. 
26 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

0

5,000

10,000

15,000

20,000

0 5,000 10,000 15,000 20,000

COMBINED DISCHARGE OF THE ANIMAS AND FLORIDA RIVERS 

 MULTIPLIED BY A FACTOR OF 2.5, 

IN CUBIC FEET PER SECOND

DISCHARGE OF THE SAN JUAN RIVER AT THE NEAR BLUFF 

GAGING STATION, IN CUBIC FEET PER SECOND

LINE OF PERFECT AGREEMENT

A.

0

5,000

10,000

15,000

20,000

San Juan River near Bluff 

2.5 x (Animas River + Florida River) 

DAILY MEAN DISCHARGE, IN CUBIC FEET PER SECOND

1928 1929 1930 1931 1932 1933 1934 1935 1936 1937

YEAR

B.

Figure 11. Comparison of the May through June daily mean discharge of the San Juan River with the combined 

previous day’s discharge of the Animas and Florida Rivers during water years 1928–37. Daily mean discharge data 

obtained electronically from U.S. Geological Survey (accessed November 22, 2000). (A) 2.5 times the combined 

previous day’s discharge of the Animas and Florida Rivers in relation to the daily mean discharge of the San Juan 

River at the near Bluff gaging station. (B) Daily mean discharge of the San Juan River at the near Bluff gaging 

station compared to 2.5 times the previous day’s combined discharge of the Animas and Florida Rivers during May 

through June of each water year. 
Colorado River Floods at Lees Ferry 27

Therefore, the combined discharge of the upper Colorado, 

Green, and San Juan Rivers contributing to the peak 

discharge of the June 1921 flood at Lees Ferry was 

approximately 167,000 ft3/s.

Revised Estimate of the Peak Discharge of the 

1921 Flood at Lees Ferry

Because the peak discharge of the June 1921 flood 

had to be the same throughout the Lees Ferry reach, 

extrapolations of the stage-discharge rating curves at the 

Lees Ferry Gage and the three staff gages in the 1-milelong reach upstream were used to establish constraints on 

the peak discharge of this flood (fig. 12). A strong reversal 

in curvature is evident in the stage-discharge rating curves 

defined by the data from the Lees Ferry, Number 4, and 

Cable Gages, and a mild reversal in curvature is evident in 

the stage-discharge rating curve defined by the data from 

the Number 1 Gage. At stages in excess of those at which 

the reversal in curvature occurs at these four gages, 

analyses of the goodness of fit of linear and power-law 

regressions indicate that the high-stage parts of the 

stage-discharge rating curves at these gages are best 

approximated by either a linear or a power-law relation, 

in which discharge is proportional to stage raised to a 

power of less than one (fig. 12).

The reversals in curvature of the stage-discharge 

data at the Lees Ferry Gage for the entire pre-dam period 

of 1921–62 and for only water-years 1921–22 both 

occur at a stage of 15 ft. Linear regressions fit to the 

1921–62 and 1921–22 data above this stage are virtually 

identical (fig. 12A), and have R2 values of 0.929 and 

0.932, respectively. Similarly, power-law regressions 

fit to the 1921–62 and 1921–22 data above this stage are 

also virtually identical (fig. 12A), and have R2 values of 

0.932 and 0.974, respectively. The similarity of the 

regressions fit to the 1921–62 and the 1921–22 data 

indicates that the upper end of the stage-discharge rating 

curve at the Lees Ferry Gage was fairly stable over time. 

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000

180,000

200,000

220,000

240,000

5 10 15 20 25 30 35 40

1921–23 DATA

LINEAR AND POWER-LAW REGRESSIONS FIT TO THE 

1921–23 DATA ABOVE A STAGE OF 16 FEET 

DISCHARGE, IN CUBIC FEET PER SECOND

STAGE AT NUMBER 4 GAGE, IN FEET

PUBLISHED PEAK DISCHARGE

OF THE 1921 FLOOD

PEAK STAGE OF THE 

1921 FLOOD

REVERSAL IN

CURVATURE

OF DATA

EXPLANATION

A. B.

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000

180,000

200,000

220,000

240,000

5 10 15 20 25 30 35 40

1921–62 DATA

LINEAR AND POWER-LAW REGRESSIONS FIT TO ALL 

THE 1921–62 DATA ABOVE A STAGE OF 15 FEET 

LINEAR AND POWER-LAW REGRESSIONS FIT TO ONLY

THE 1921–22 DATA ABOVE A STAGE OF 15 FEET 

DISCHARGE, IN CUBIC FEET PER SECOND

STAGE AT LEES FERRY GAGE, IN FEET

PUBLISHED PEAK DISCHARGE

OF THE 1921 FLOOD

PEAK STAGE OF THE 

1921 FLOOD

PEAK STAGE OF THE 1884 FLOOD

REVERSAL IN

CURVATURE

OF DATA

EXPLANATION

Figure 12. Linear and power-law extrapolations of the stage-discharge rating curves at the Lees Ferry, Number 4, Cable, and Number 1 Gages. 

(A) Rating curve at the Lees Ferry Gage defined by the 4,114 non-ice-affected discharge measurements made during water years 1921–62. Stage is relative 

to the Dugway Gage and Lees Ferry Gage datum of September 21, 1921 (see Appendix B). Shown for comparison are the peak stages of the 1921 and 

1884 floods and the published peak discharge of the 1921 flood. (B) Rating curve at the Number 4 Gage defined by the 209 discharge measurements made 

during water years 1921–23 that included stage observations at the Number 4 Gage. Stage is relative to the Number 4 Gage datum of September 29, 1921

(see Appendix B). Shown for comparison are the peak stage and published peak discharge of the 1921 flood. Stage is relative to the Number 1 Gage datum 

of September 26, 1921 (see Appendix B). Shown for comparison are the peak stage and published peak discharge of the 1921 flood.
28 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000

180,000

200,000

220,000

240,000

10 15 20 25 30 35 40

1924–62 DATA

LINEAR AND POWER-LAW REGRESSIONS FIT TO THE 

1924–62 DATA ABOVE A STAGE OF 26 FEET

DISCHARGE, IN CUBIC FEET PER SECOND

STAGE AT CABLE GAGE, IN FEET

PUBLISHED PEAK DISCHARGE

OF THE 1921 FLOOD

PEAK STAGE OF THE

1921 FLOOD

REVERSAL IN

CURVATURE

OF DATA

EXPLANATION

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000

180,000

200,000

220,000

240,000

5 10 15 20 25 30 35 40

1921–25 DATA

LINEAR AND POWER-LAW REGRESSIONS FIT TO THE 

1921–25 DATA ABOVE A STAGE OF 20 FEET 

DISCHARGE, IN CUBIC FEET PER SECOND

STAGE AT NUMBER 1 GAGE, IN FEET

PUBLISHED PEAK DISCHARGE

OF THE 1921 FLOOD

PEAK STAGE OF THE 

1921 FLOOD

MILD REVERSAL IN

CURVATURE

OF DATA

EXPLANATION

C. D.

Figure 12—Continued. Linear and power-law extrapolations of the stage-discharge rating curves at the Lees Ferry, Number 4, Cable, and Number 1 

Gages. (C) Rating curve at the Cable Gage defined by the 3,196 discharge measurements made during water years 1924–62 that included stage 

observations at the Cable Gage. Stage is relative to the revised Cable Gage datum of October 12, 1924 (see Appendix B). Shown for comparison are the peak 

stage and published peak discharge of the 1921 flood. (D) Rating curve at the Number 1 Gage defined by the 136 discharge measurements made during 

water years 1921–22 that included stage observations at the Number 1 Gage and by the 81 discharge measurements during water years 1924–25 on days 

with stage observations at the Number 1 Gage. Stage is relative to the Number 1 Gage datum of September 26, 1921 (see Appendix B). Shown for 

comparison are the peak stage and published peak discharge of the 1921 flood. 

This result suggests that extrapolation of either the data 

from 1921–62 or only the data from 1921–22 should 

provide a reasonably good estimate of the peak discharge 

of the 1921 flood. The similar goodness-of-fit values for 

the linear and power-law regressions suggest that the 

upper end of the stage-discharge rating-curve at the Lees 

Ferry Gage can be approximated either as a line or as a 

power-law function. Linear extrapolations of both the 

1921–62 and 1921–22 data suggest that the peak 

discharge of the 1921 flood was about 182,000 ft3/s, and 

power-law extrapolations of both the 1921–62 and 1921–

22 data suggest that the peak discharge of the 1921 flood 

was about 168,000 ft3/s.

The reversal in curvature of the 1921–23 stagedischarge data at the Number 4 Gage occurs at a stage of 

16 ft on this gage. The linear regression fit to the data 

above this stage has an R2 value of 0.971; the power-law 

regression fit to the data above this stage has an R2 value 

of 0.975. As at the Lees Ferry Gage, the similar goodnessof-fit values for the linear and power-law regressions 

suggest that the upper end of the stage-discharge ratingcurve at the Number 4 Gage can be approximated either 

as a line or as a power-law function. Linear extrapolation 

of the data suggest that the peak discharge of the 

1921 flood was about 156,000 ft3/s, and power-law 

extrapolation of the data suggest that the peak discharge 

of the 1921 flood was about 147,000 ft3/s (fig. 12B). 

The reversal in curvature of the 1924–62 stagedischarge data at the Cable Gage occurs at a stage of 

26 ft on this gage. Linear and power-law regressions 

fit to the data above this stage both have R2 values of 

0.971. The identical goodness-of-fit values for the linear 

and power-law regressions suggest that the upper end of 

the stage-discharge rating-curve at the Cable Gage can be 

approximated either as a line or as a power-law function. 

Linear extrapolation of the data suggest that the peak 

discharge of the 1921 flood was about 145,000 ft3/s, and 

power-law extrapolation of the data suggest that the peak 

discharge of the 1921 flood was about 143,000 ft3/s 

(fig. 12C).
Colorado River Floods at Lees Ferry 29

 The mild reversal in curvature of the 1921–25 

stage-discharge data at the Number 1 Gage occurs at a 

stage of 20 ft on this gage. The linear regression fit to the 

data above this stage has an R2 value of 0.982; the 

power-law regression fit to the data above this stage 

has an R2 value of 0.983. As at the other gages, the 

similar goodness-of-fit values for the linear and powerlaw regressions suggest that the upper end of the stagedischarge rating-curve at the Number 1 Gage can be 

approximated either as a line or as a power-law function. 

Linear extrapolation of the data suggest that the peak 

discharge of the 1921 flood was about 187,000 ft3/s, and 

power-law extrapolation of the data suggest that the peak 

discharge of the 1921 flood was about 174,000 ft3/s 

(fig. 12D). 

Linear and power-law extrapolations of the stagedischarge rating curves at these four gages by means of 

best-fit regressions lead to an estimated peak discharge of 

the June 1921 flood that ranges between 143,000 and 

187,000 ft3/s. This discharge range is somewhat wider 

than the 163,000 to 188,000 ft3/s range that was based 

on the data from the Grand Canyon gaging station and 

other upstream and downstream gaging stations described 

above, but the upper ends of the ranges are nearly equal. 

Based on all of the data from the Lees Ferry and other 

gaging stations, the best estimate of the peak discharge 

of the June 1921 flood at Lees Ferry is 170,000 ft3/s, at 

two significant figures (fig. 13). Given the range in the 

estimates of the peak discharge at the Lees Ferry and 

other gaging stations, the uncertainty in this value is 

approximately 20,000 ft3/s. Our estimate of 170,000± 

20,000 ft3/s is therefore indistinguishable from both the 

original USGS unpublished estimate of 174,000 ft3/s 

and the original USGS published value of "about 

190,000 ft3/s." This estimate of 170,000±20,000 ft3/s is 

incompatible, however, with the 1939 upward revision of 

220,000 ft3/s based on Gatewood and Hunter’s 1938 

study. 

Estimate of the Peak Discharge of the 

1884 Flood at Lees Ferry

Estimation of the peak discharge of the 1884 

flood depends on accurate determination of maximum 

flood stage and appropriate extrapolation of stagedischarge rating curves to higher elevations. The 

original unpublished estimate of the peak discharge 

of this flood at Lees Ferry was between 210,000 and 

250,000 ft3/s (G.C. Stevens, unpublished U.S. Geological 

Survey memorandum, May 28, 1925), and this estimate 

was revised by Gatewood and Hunter in 1938 to 

300,000 ft3/s. They made this revision because they had 

revised the magnitude of the 1921 flood upward; this 

revision resulted in a change in the stage-discharge rating 

curve which, in turn, necessitated revision of the 1884 

estimate. Reevaluation of stage-discharge data indicate 

that the probable range of the peak discharge of this flood 

was between 199,000 and 228,000 ft3/s. 

The Original U.S. Geological Survey Estimate of the 

Peak Discharge of the 1884 Flood

The original USGS estimate of the peak discharge 

of the July 1884 flood was based on extrapolation of 

the stage-discharge rating curve at the Lees Ferry 

Gage, and on extrapolation of crude stage-discharge 

relationships developed for the Colorado River at three 

sites downstream in Grand Canyon (G.C. Stevens, 

unpublished U.S. Geological Survey memorandum,

May 28, 1925). These values ranged from 210,000 ft3/s 

at River-mile 233.7 (fig. 1A) to 250,000 ft3/s at Lees 

Ferry.

For example, Stevens calculated that the 

discharge of the July 1884 flood at River-mile 233.7 was 

approximately 210,000 ft3/s, based on data collected on 

October 8, 1923, during the USGS Birdseye Expedition 

(Westwood, 1992). Stevens wrote:

During the survey of the Colorado River in 1923, an 

exceptionally high flood mark consisting of a pile of 

old drift was noted at Mile 233.7. The following 

information has been furnished by Mr. Herman Stabler:

High-water mark, date unknown - - - - - - - - 1298.4’

High-water mark, Sept, 20–23, 1923 - - - - - - 1275.1’

Water level, October 8, 1923

(date of survey) - - - - - - - - - - - - - - - - - 1244.9’

The flow at the Bright Angel station [the Grand 

Canyon gaging station] a few days before October 8, 1923, 

was 13,000–14,000 second-feet [ft3

/s]. The flow during 

the September flood was 112,000 second-feet. [This flood 

largely originated in the Little Colorado River.] Using 

these two points and extending as a straight line to 

elevation 1298.4’ gives 190,000 second-feet. By a curved 

extension, from 210,000 to 220,000 may be obtained. Accept 

210,000 second-feet for the present. If the crest of the 

September, 1923, flood at Mile 233.7 was lower in 

discharge than at Bright Angel [i.e., the Grand Canyon 

gaging station] the extension would give a lower figure for 

the high-water mark.
30 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000

180,000

200,000

220,000

240,000

260,000

5 10 15 20 25 30 35

WATER-YEARS 1921–22 DATA 

RATING CURVE 

DISCHARGE, IN CUBIC FEET PER SECOND

STAGE AT LEES FERRY GAGE, IN FEET

RANGE OF UNCERTAINTY IN THE ESTIMATED PEAK 

DISCHARGE OF THE 1884 FLOOD

RANGE OF UNCERTAINTY IN THE ESTIMATED PEAK 

DISCHARGE OF THE 1921 FLOOD

EXPLANATION

PEAK STAGE OF THE 1921 FLOODPEAK STAGE OF THE 1884 FLOOD

PEAK DISCHARGE OF

 THE 1921 FLOOD

PEAK DISCHARGE OF

 THE 1884 FLOOD

Figure 13. Stage-discharge rating curve used in this study for the period between May 8, 1921, and June 25, 1921. This rating curve 

consists of a smoothed curve fit to the discharge measurements from water years 1921 and 1922, and a linear extrapolation to the 

170,000±20,000 cubic feet per second peak of the 1921 flood at a stage of 26.5 feet, and the 210,000±30,000 cubic feet per second peak of 

the 1884 flood at a stage of 31 feet.
Colorado River Floods at Lees Ferry 31

This high driftwood deposit is located at River-mile 

233.7 on the right bank at the lower end of 234-mile rapid 

(fig. 14A), and was revisited on November 9, 2002. An old 

rusty metal can was found placed adjacent to the top of the 

deposit (fig. 14B). This can was likely left by the Birdseye 

Expedition rodman; the presence of this can suggests 

strongly that this deposit was the one surveyed in 1923 by 

the Birdseye Expedition. As indicated in the 1925 Stevens 

memorandum, the great height of this deposit above the 

112,000 ft3/s water-surface elevation surveyed in 1923 

suggests that this deposit was formed during a flood 

with a peak discharge much greater than that of the 

170,000 ft3/s 1921 flood. The presence of milled lumber 

(fig. 14C) indicates that this deposit was formed during a 

flood that occurred after white settlement of the Colorado 

River Basin upstream from Grand Canyon. Thus, it is 

likely that this driftwood deposit was formed during the 

1884 flood. 

Revised Estimate of the Peak Discharge of the 

1884 Flood at Lees Ferry

Linear and power-law extrapolation of the stagedischarge rating curve at the Lees Ferry Gage suggests 

that the peak discharge of the July 1884 flood was 

between 199,000 and 228,000 ft3/s (fig. 12A). This 

discharge range brackets the estimated 214,000 ft3/s peak 

discharge for this flood at the Grand Canyon gaging 

station (fig. 9D), and overlaps with the range of 210,000 to 

250,000 ft3/s originally estimated by the USGS for the 

peak discharge of this flood at various sites between Lees 

Ferry and River-mile 233.7. Based on the estimates at 

Lees Ferry, the revised estimate at the Grand Canyon 

gaging station, and the previous estimate made at Rivermile 233.7, the peak discharge of the July 1884 flood, at 

two significant figures, was approximately 

210,000±30,000 ft3/s at Lees Ferry and through Grand 

Canyon (fig. 13).

Implications for Previous Estimates of 

Paleoflood Discharges

O’Connor and others (1994) determined that during 

the last 4,500 years, 15 floods at Lees Ferry had peak 

discharges larger than 190,000 ft3/s. Ten of these floods 

had peak discharges larger than 240,000 ft3/s during the 

last 2,100–2,300 years, and one flood that occurred 

1,200–1,600 years ago had a peak discharge exceeding 

490,000 ft3/s. On the basis of the data presented in this 

study, the discharges of these paleofloods are 

overestimated.

O’Connor and others (1994) conducted their 

paleoflood study in the reach between the Lower Staff 

Gage and Cathedral Wash, and the principal stratigraphic 

evidence was at "Axehandle Alcove," 2.2 miles 

downstream from the Lees Ferry Gage (figs. 1B and 4A). 

They speculated that a flood deposit at the top of their 

section, "G1," was deposited during the July 1884 flood. 

They estimated their paleoflood discharges by using a 

step-backwater model with the published 300,000 ft3/s 

peak discharge of the July 1884 flood as a guide. In their 

calculations, they did not take into account the fact that the 

water-surface slopes in this reach decrease with increasing 

stage as backwatered flow conditions develop in the reach. 

In fact, water-surface slopes computed by O’Connor and 

others (1994) steepen with increasing stage. As shown in 

fig. 4A, it is more likely that "G1" was deposited near the 

170,000 ft3/s peak discharge of the June 1921 flood.

Because the Axehandle Alcove site is just 

downstream from the Lower Staff Gage, discharges can 

be directly assigned to these flood deposits on the basis of 

the data presented in this paper. On May 17, 1941, the 

discharge of the Colorado River was approximately 

120,000 ft3/s. The stage associated with this flood was 

observed to be 28.1 ft on the Lower Staff Gage, which 

corresponds to an elevation of 3,122.0 above sea level. 

Extrapolation of the 120,000 ft3/s water-surface profile 

downstream indicates that the elevation of the May 17, 

1941, water surface was approximately 3,120.1 ft above 

sea level at the Axehandle Alcove site. This elevation 

is roughly equivalent to the elevation of the base of 

O’Connor and others (1994) stratigraphic section. If the 

channel geometry in this reach in 1941 were similar to its 

geometry 3,000–4,500 years ago, the lowermost flood 

deposit preserved in Axehandle Alcove was deposited by 

a flood with a peak discharge just above 120,000 ft3/s.

On the basis of fig. 4A, the elevation of the water 

surface during the peak of the 1921 flood at Axehandle 

Alcove was probably about 3,130.6 ft above sea level. 

This elevation is about 1.3 ft higher than the elevation of 

the deposit "G1" in fig. 3 in O’Connor and others (1994), 

and is similar to the highest elevation reported in the reach 

for "G1." Thus, it is likely that "G1" was not deposited 

during the 1884 flood, but rather during the 1921 flood. 
32 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

A.

B.

C.

Figure 14. River-mile 233.7 driftwood deposit. Photographs taken on November 9, 2002, by D.J. Topping of the U.S. Geological Survey.

(A) View from left to right bank of the high driftwood deposit at River-mile 233.7 surveyed by the Birdseye Expedition on October 8, 1923. 

(B) Old rusty metal can placed adjacent to the top of the deposit, likely by the Birdseye Expedition rodman. (C) Milled lumber within the 

driftwood deposit. 
Computation of the Continuous Record of Instantaneous Discharge at Lees Ferry for Water Years 1921–2000 33

On the basis of fig. 4A, the water-surface elevation during 

the peak of the 1884 flood at Axehandle Alcove was 

probably about 3,136.5 ft above sea level, 6.6 ft higher 

than deposit "G1." 

The highest deposit in Axehandle Alcove, the 

1,200–1,600 year-old crevice deposit, is about 16.4 ft 

higher than deposit "G1," and about 9.8 ft higher than the 

likely peak stage of the 1884 flood. Given that the water 

surface was probably almost flat between the site of the 

Lees Ferry Gage and Axehandle Alcove (fig. 4A), the 

peak stage of this 1,200–1,600 year-old flood at the 

site of the Lees Ferry Gage would have been about 

41 ft. Extrapolation of the stage-discharge rating curve 

in fig. 13, therefore, suggests that the peak discharge of 

the paleoflood that deposited the crevice deposit was 

about 300,000 ft3/s. Therefore, based on these revised 

paleoflood discharges and the stratigraphy of O’Connor 

and others (1994), the deposits of 15 paleofloods during 

the last 4,500 years with peak discharges larger than 

120,000 ft3/s are preserved at Axehandle Alcove. Of these 

paleoflood deposits, 10 are preserved from floods during 

the last 2,100–2,300 years with peak discharges in excess 

of about 140,000–150,000 ft3/s, and one deposit is 

preserved from a flood 1,200–1,600 years ago with a peak 

discharge in excess of about 300,000 ft3/s.

COMPUTATION OF THE CONTINUOUS 

RECORD OF INSTANTANEOUS 

DISCHARGE AT LEES FERRY FOR

WATER YEARS 1921–2000

All of the raw data collected by the USGS at Lees 

Ferry were retrieved from storage in the Federal Records 

Centers (Appendix A). After this retrieval, computation of 

the continuous record of instantaneous discharge was a 

multi-step process that took almost 4 years to complete. 

Methods Used by the U.S. Geological 

Survey to Compute the Previously 

Published Daily Mean Discharge 

Record for Water Years 1921–86

The first step in this process was to review the 

methods used by the USGS to compute the previously 

published daily mean discharge record. During water 

years 1921–86, the USGS computed discharge at the Lees 

Ferry gaging station by the standard method of using a 

stage-discharge rating curve to convert measured stage to 

discharge (Rantz and others, 1982). Prior to January 19, 

1923, stage was observed twice daily at the various staff 

gages in the reach between the Upper Cableway and the 

Lees Ferry Gage. After this date, stage was measured 

continuously by the float in the stilling well and was 

recorded on an analog strip-chart recorder. The chart 

recorder was replaced as the primary record of stage on 

February 2, 1967, when a digital recorder was installed in 

the gage. After this date, the analog chart record was used 

only as a back-up in case the digital recorder failed. From 

February 2, 1967, through February 28, 1974, stage 

was recorded digitally every 2 hours. Subsequently, 

stage was recorded digitally every 30 minutes to increase 

the accuracy of the published daily mean discharges. 

During each water year, between 5 and 200 discharge 

measurements were made (fig. 15), and used to construct 

the stage-discharge rating curves and to compute shift 

curves. Thirty-six different rating curves were used to 

compute the published record at Lees Ferry from June 13, 

1921, through September 30, 1986 (table 2). 

During most days in the pre-dam era, daily mean 

discharge was computed by applying a stage-discharge 

rating curve to a daily mean stage. During days when the 

discharge was variable, the day would be first subdivided 

into portions of comparable stage. Then, the mean stage 

for each of these portions would be converted into a mean 

0

50

100

150

200

1920 1930 1940 1950 1960 1970 1980 1990

NUMBER OF DISCHARGE MEASUREMENTS 

MADE DURING EACH WATER YEAR

WATER YEAR

Figure 15. Number of discharge measurements made at Lees Ferry during 

each water year between 1921 and 1986.
34 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

Table 2. Stage-discharge rating curves used to construct the published record of daily mean discharge at the Lees Ferry gaging station during water years 1921 

through 1986

Rating

 curve name Dates used Rating curve name Dates used

15-03-38 6-13-1921 through 7-10-1921,8-23-1921, 10-15-29 10-01-1928 through 5-29-1929

5-21-1922 through 6-2-1922, 10-18-29 5-30-1929 through 9-30-1929

5-01-1923 through 5-31-1923 10-15-32 3-20-1932 through 6-12-1933, 

11-12-23 7-11-1921 through 8-22-1921, 6-21-1933 through 9-30-1934

8-24-1921 through 9-30-1921 11-9-33 6-13-1933 through 6-20-1933

11-16-23 10-01-1921 through 5-20-1922, 3-6-36 10-01-1934 through 9-30-1936

6-03-1922 through 7-10-1922 1-6-38 10-01-1936 through 2-28-1938

11-20-23 7-11-1922 through 1-18-1923 1-14-39 3-01-1938 through 3-20-1939

1-08-24 1-19-1923 through 4-30-1923, 9-5-39 3-21-1939 through 10-31-1939

6-01-1923 through 9-30-1923 9-10-40 11-01-1939 through 1-20-1943

10-30-24 10-01-1923 through 9-30-1924 8-27-43 1-21-1943 through 4-24-1948, 

2-04-26 10-01-1924 through 9-19-1925 1-01-1949 through 4-23-1949

2-15-26 9-20-1925 through 9-30-1925 7-23-48 4-25-1948 through 12-31-1948

12-06-26 10-01-1925 through 5-06-1926, 7-25-49 4-24-1949 through 4-30-1952, 

noon on 2-17-1927 through 4-20-1927 1-01-1953 through 12-31-1955

12-09-26 5-07-1926 through 5-30-1926 8-5-52 5-01-1952 through 12-31-1952

12-08-26 5-31-1926 through 12-16-1926 RT 1 1-01-1956 through 5-31-1963

2-24-26 12-17-1926 through noon on 2-17-1927 RT 2 6-01-1963 through 9-30-1968

12-14-27 4-21-1927 through 7-01-1927 RT 3 10-01-1968 through 9-30-1972

12-15-27 7-02-1927 through 9-30-1927, RT 4 10-01-1972 through 4-30-1983

5-15-1928 through 9-30-1928 RT 5 5-01-1983 through 10-18-1984

12-10-28 10-01-1927 through 2-29-1928 RT 6 10-19-1984 through 9-30-1986

12-11-28 3-01-1928 through 5-14-1928

11938 revision by Gatewood and Hunter.

discharge, which would then be used to compute a daily 

mean discharge. After the closure of Glen Canyon Dam, 

when the discharge each day became highly variable due 

to hydroelectric-power generation, the discharge record 

for all days would be computed by this subdivision 

method. 

Since May 7, 1926, shifts have been applied to the 

measured stage prior to computing the discharge with a 

rating curve (table 3). This standard USGS method is 

known as the "shifting-control" method (described on 

page 25 in Kennedy, 1984). Shift curves for each year 

were based on the smoothed disagreement between the 

measured discharge and the discharge predicted by the 

stage-discharge rating curve. For example, if, over 

a given period, the observed stages during discharge 

measurements were consistently 0.1 foot higher than the 

stages associated with those discharges on the rating 

curve, 0.1 foot would be subtracted from the measured 

stages for that period prior to computing discharge with 

the stage-discharge rating curve. Thus, use of the shiftingcontrol method at Lees Ferry allowed the published 

discharges to agree more closely with the measured 

discharges.

Glen Canyon Dam has greatly reduced the annual 

range of water temperature in the river at Lees Ferry. 

Prior to closure of the dam, ice routinely made the 

computation of discharge difficult at Lees Ferry during the 

winter (figs. 16A–B). During extremely cold periods, the 

river would completely freeze over, the float would freeze 

in the well, and no stage record would be obtained. More 

typically, the river would freeze at night, and the stage 

would increase slowly. The ice would then begin to melt 

the next morning, and the stage would decrease quickly 

around noon (fig. 16C). Other types of ice effect at the 
Computation of the Continuous Record of Instantaneous Discharge at Lees Ferry for Water Years 1921–2000 35

Lees Ferry Gage included: (1) the formation of large ice 

jams on the riffle at the Paria River gravel bar that would 

back up large quantities of water and cause rises in stage 

by a foot or more, until the ice jams failed, and (2) the 

complete freezing of the surface of the river, during which 

the float would be frozen in the stilling well. During 

periods when the Lees Ferry Gage was affected by ice, 

daily mean discharge was typically computed by 

subtracting the sum of the measured and estimated 

inflows of water between the Lees Ferry and the Grand 

Canyon gaging stations from the daily mean discharge 

computed at the Grand Canyon gaging station. 

Between 1931 and 1945, the record at the Grand 

Canyon gaging station was routinely used to modify the 

discharge computed at Lees Ferry during periods of 

higher flow in the spring and summer (Dickinson, 1944). 

During higher flows, slightly more water was sometimes 

computed passing Lees Ferry than was computed passing 

the Grand Canyon gaging station. This artifact was largely 

due to instability in the Lees Ferry stage-discharge rating 

curve above the reversal in curvature at a stage of 15 ft. 

Beginning in water year 1931, the discharge record at the 

Grand Canyon gaging station (because of its more stable 

stage-discharge rating curve) was used to reduce the 

computed discharges at Lees Ferry in order to bring them 

into agreement with those computed at the Grand Canyon 

gaging station (fig. 17). Although this was done almost 

every year from 1931 through 1945, these reductions were 

the most extensive during water years 1934 and 1936. 

During water year 1934, the daily mean discharges 

computed at Lees Ferry were reduced by 1 percent from 

May 9 through May 28. During water year 1936, the daily 

mean discharges computed at the gage were reduced by 1 

percent from May 20 through June 15, 2 percent from 

June 16 through June 20, 3 percent from June 21 through 

June 30, 2 percent from July 1 through July 4, and 1 

percent from July 5 through July 25. 

Prior to 1931, it was not the policy of the USGS 

to use the record at the Grand Canyon gaging station to 

modify the discharge record at Lees Ferry for periods 

longer than a few days, except during periods of ice at 

Lees Ferry. The one extended period prior to 1931 when 

the record at the Grand Canyon gaging station should 

have been used to modify the discharges at the Lees Ferry 

gaging station was water year 1929. Between March 6 

and September 30, 1929, 2.7 percent more water was 

computed passing Lees Ferry than was computed passing 

the Grand Canyon gaging station (after corrections had 

been made for the tributary inflow of water between the 

two sites). In an unpublished USGS memorandum dated 

December 30, 1929, W.E. Dickinson computed that the 

discharges at the Lees Ferry gaging station were 2.6 

percent too high from March 6 through March 21, 3.3 

percent too high from April 2 through April 15, 3.4 

percent too high from April 15 though May 2, 2.7 percent 

too high from May 2 through July 16, 3.5 percent too high 

from July 16 through August 26, 4.9 percent too high 

from August 26 through September 19, and 2.3 percent 

too high from September 19 through September 30 

(Appendix C). Although the cause of the discharge errors 

at the Lees Ferry gaging station between March and 

September 1929 were never officially determined, there 

are three possible explanations. The Upper Cableway was 

replaced in March 1929, and this could have affected the 

discharge measurements. The current meters used at Lees 

Ferry were changed on February 15, and this change in 

current meters could also have affected the discharge 

measurements. Finally, in March 1929, the abandoned 

dugway road on the left side of the gage was filled with 

rocks (Appendix B, fig. B6). This fill prevented water 

from flowing around both sides of the gage house during 

high discharges and could have affected the stage 

measurements at the gage. 

Table 3. Dates when the shifting-control method was used to construct 

the published record of daily mean discharge at the Lees Ferry gaging 

station during water years 1921 through 1986

Date

5-07-1926 through 5-8-1926

5-31-1926 through 6-16-1926

40-8-1928 through 4-22-1928

5-07-1928 through 5-10-1928

5-15-1928 through 6-12-1933

6-21-1933 through 10-12-1936

10-28-1936 through 11-14-1936

12-24-1936 through 7-18-1937

8-03-1937 through 9-30-1940

5-17-1941 through 5-19-1941

5-30-1941 through 9-30-1968

12-14-1968 through 9-30-1973

6-04-1983 through 6-27-1983

8-03-1983 through 10-18-1984

5-09-1986 through 9-30-1986
36 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

A.

B.

C. STAGE INCREASES DUE TO 

ICE FORMING ON HYDRAULIC CONTROL

STAGE CORRECTED FOR ICE EFFECT

noon midnight noon midnight noon

6.0

7.0

DATE

STAGE, IN FEET

1-7-1947 1-8-1947 1-9-1947

 ICE 

MELTS

 ICE 

MELTS

 ICE 

MELTS

0

10

20

30

40

50

60

70

1920 1930 1940 1950 1960 1970 1980 1990

NUMBER OF DAYS 

AFFECTED BY ICE

WATER YEAR

CLOSURE OF 

GLEN CANYON DAM

Figure 16. The pre-dam effects of ice at the Lees Ferry gaging station. (A) Number of days during each water year between 1922 

and 1986 when the stage measured at the Lees Ferry Gage was affected by ice. (B) View of the ice breaking up in front of the 

Lees Ferry Gage from the right bank. Photograph taken by the U.S. Geological Survey, probably between December 17 and 23, 

1927. (C) The most typical type of ice effect, as shown by the January 7–9, 1947, stage-recorder graph from the Lees Ferry Gage. 

Dashed line is the stage corrected for the effect of the ice (this is the stage that was digitized in this study). 
Computation of the Continuous Record of Instantaneous Discharge at Lees Ferry for Water Years 1921–2000 37

Methods Used in this Study to 

Compute the Continuous Record of 

Instantaneous Discharge for 

Water Years 1921–86

Following review of the methods used by the 

USGS to compute the daily mean discharge record, 

the first step in computing the continuous record of 

instantaneous discharge was to construct a continuous 

record of instantaneous stage from May 8, 1921, through 

September 30, 1986, relative to the datum of the Dugway 

and Lees Ferry Gages established on September 21, 1921. 

For the period prior to installation of the continuous stage 

recorder on January 19, 1923, this step involved first 

entering the stages measured at the three principal staff 

gages in the reach (i.e., the LaRue Gage, Number 1 Gage, 

and Dugway Gage) into computer files from the original 

USGS field notebooks (see Appendix B for additional 

background information on these staff gages). Each staff 

gage had a different period of record and datum; the 

LaRue Gage was installed on May 8, 1921, and was 

destroyed during the June 1921 flood; the Number 1 Gage 

was installed at the site of the LaRue Gage on June 24, 

1921, but at a different datum; the Dugway Gage was 

installed on August 5, 1921, near the future location 

and at the same datum as the Lees Ferry Gage. Thus, 

for the period prior to the beginning of record at the 

Dugway Gage on August 14, 1921, the stages measured 

on the LaRue and Number 1 Gages were converted 

into equivalent stages on the Dugway Gage. This was 

accomplished by first converting the stages measured on 

the LaRue Gage into stages on the Number 1 Gage 

by a process described in Appendix D. Then, the stages 

measured on either the LaRue or Number 1 Gages prior 

to August 14, 1921, were converted into equivalent stages 

on the Dugway Gage on the basis of the relationship 

between the Number 1 and Dugway Gages developed 

by the USGS on January 25, 1922. 

For the period after installation of the Lees Ferry 

Gage on January 19, 1923, with its continuous strip-chart 

stage recorder, construction of the continuous record 

of instantaneous stage involved digitizing all of the 

stage-recorder graphs between January 19, 1923, and 

September 30, 1986, such that all major breaks in 

slope of the continuous stage record were captured. 

These digitized stage records were then corrected for time 

and pen errors. During periods of ice effect, the stagerecorder graphs were digitized so that most, if not all, of 

the ice effect was removed from the stage record. For 

example, the period in fig. 16C was digitized by following 

the dashed pencil line that was originally drawn on the 

record by the USGS to correct for the ice-driven rises in 

stage. During the two periods when large floods on the 

Paria River caused backwatered flow conditions in the 

Colorado River at the Lees Ferry Gage (on October 5, 

1925, and on August 2, 1929; Appendix B), the resulting 

increased stages were removed from the digitized stage 

record by the same method. To augment and check the 

stage records digitized from the stage-recorder graphs, the 

digital stage-recorder records from February 2, 1967, 

through September 30, 1986, were also entered into 

computer files. 

The second step in this process was to convert the 

continuous record of instantaneous stage into a continuous 

record of instantaneous discharge by using the stagedischarge rating curves and shift curves originally used by 

the USGS to compute the published daily mean 

discharges. To accomplish this, all of the rating curves and 

shift curves were entered into computer files and then 

were used to compute instantaneous discharge for the 

period from May 8, 1921, through September 30, 1986. 

Finally, the discharges at Lees Ferry were reduced to 

0

20

40

60

80

100

1920 1930 1940 1950 1960 1970 1980 1990

NUMBER OF DAYS WHEN DISCHARGE 

WAS MODIFIED ON THE BASIS OF THE RECORD AT

 THE GRAND CANYON GAGING STATION

WATER YEAR

WATER YEAR 1934

WATER YEAR 1936

Figure 17. Number of days when the daily mean discharge record at Lees 

Ferry was modified on the basis of the discharge record at the Grand 

Canyon gaging station. This figure includes days when discharge was 

modified because of ice conditions and other reasons, such as during 1934 

and 1936 when the computed discharge on many days in the spring and 

summer was reduced at Lees Ferry. 
38 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

match the discharges measured at the Grand Canyon 

gaging station (corrected for tributary inflow) during those 

non-ice-affected periods when this was originally done by 

the USGS (see previous section).

The methods used in this study deviated from those 

used by the USGS during the original computation of the 

daily mean discharge record in four respects. First, in this 

study, discharges at the Grand Canyon gaging station were 

not used to correct any discharges at Lees Ferry on days 

affected by ice. All ice corrections were made during the 

digitizing of the stage-recorder graphs. Second, the 1938 

revised stage-discharge rating curve and shift curves of 

Gatewood and Hunter were used during the computation 

of discharge on the following days: June 26, 1921–

September 10, 1921, April 22, 1922–July 17, 1922, 

April 17, 1923–July 31, 1923, April 8, 1924–July 16, 

1924, April 1, 1925–July 21, 1925, and April 16, 1926–

July 20, 1926. Unlike their method of estimating the peak 

discharges of the 1884 and 1921 floods, which was 

shown to be in error, Gatewood and Hunter’s method 

of computing discharge during these six periods results 

in computed discharges that agree closely with those 

measured at the Upper Cableway. In fact, the discharges 

computed by Gatewood and Hunter’s approach during 

these periods agree much more closely with the measured 

discharges than do the discharges computed by means of 

the original USGS rating curves without shifts. Third, in 

order to compute discharge from May 8, 1921, through 

June 25, 1921, a smoothed stage-discharge rating curve 

was fit to the discharge-measurement data from water 

years 1921 and 1922. This rating curve was extended 

linearly from 120,000 ft3/s through the 170,000 ft3/s 

discharge determined in this study as the best value for the 

peak discharge of the June 1921 flood (fig. 13). Because 

of the uncertainty associated with the datum of the LaRue 

Gage, the USGS never published daily mean discharges 

for the period prior to June 13, 1921. The discharges 

computed in this study from May 8 through June 12, 

1921, however, are probably reasonable because they 

compare favorably with both (1) the combined discharges 

of the upper Colorado, Green, and San Juan Rivers, and 

(2) the discharge of the Colorado River at the Yuma 

gaging station (fig. 18). In fig. 18, the combined daily 

mean discharge of the upper Colorado, Green, and San 

Juan Rivers was computed by shifting the measured daily 

mean discharge record at the Colorado River near Fruita 

gaging station +2 days, shifting the measured daily mean 

discharge record at the Green River gaging station +1 day, 

and estimating the daily mean discharge of the San Juan 

River at the near Bluff gaging station (by the same 

approach as in fig. 11). The daily mean discharge record 

at the Yuma gaging station has been shifted -9 days to 

account for the approximate travel time of the flood peak 

between Lees Ferry and Yuma. Fourth, in order to 

maintain consistency in this data set, the discharges at 

Lees Ferry during March through September, 1929, were 

also reduced by the amounts stated in W.E. Dickinson’s 

December 30, 1929, memorandum (Appendix C). 

0

50,000

100,000

150,000

200,000

APRIL MAY JUNE JULY AUGUST

COLORADO RIVER AT LEES FERRY 

COMBINED UPPER COLORADO, GREEN, AND 

SAN JUAN RIVERS 

COLORADO RIVER AT THE YUMA GAGING STATION

DISCHARGE, IN CUBIC FEET PER SECOND

RANGE OF UNCERTAINTY IN HIGH DISCHARGES

AT THE YUMA GAGING STATION 

EXPLANATION

Figure 18. Comparison of the computed instantaneous discharge for 

April–August 1921 at Lees Ferry with (1) the combined daily mean 

discharge of the upper Colorado, Green, and San Juan Rivers, and (2) the 

daily mean discharge of the Colorado River at the Yuma gaging station. 

The gray shaded region covers the range of uncertainty in discharges in 

excess of about 110,000 cubic feet per second at the Yuma gaging station 

(described above in the text).
Computation of the Continuous Record of Instantaneous Discharge at Lees Ferry for Water Years 1921–2000 39

The final step in the construction of the 1921–86 

continuous record of instantaneous discharge was quality 

control; daily mean discharges were computed from this 

continuous time series of instantaneous discharge and 

compared with the daily mean discharges previously 

published by the USGS. All periods during which the 

computed daily mean discharges deviated from the 

published daily mean discharges by 1 percent or more for 

more than several days were examined for errors. During 

this process, the major digitizing errors were found and 

corrected. In addition, errors in the published daily mean 

discharge data were also identified. The most common 

type of error found was a transcription or copying error, 

where either stage or discharge was incorrectly written or 

copied either on the graph or in the final records (fig. 19). 

The record for days with substantial ice effects could not 

always be corrected because the published daily mean 

discharges on these days were actually based on the 

record from the Grand Canyon gaging station. Therefore, 

some disagreement still exists between the instantaneous 

discharges in the continuous record and the published 

daily mean discharges on days of extreme ice effect. 

Appendix E contains a complete listing of the causes of all 

disagreements greater than or equal to 5 percent between 

the daily mean discharges computed from the continuous 

record of instantaneous discharge and the published daily 

mean discharges. 

Average disagreement between the daily mean 

discharges computed from the continuous record of 

instantaneous discharge and the published daily mean 

discharges was found to be only +0.055 percent for the 

entire period from water year 1921 through water year 

1986 (fig. 20). In fig. 20, percent disagreement is defined 

as 

(1)

where Qcomp = the daily mean discharge computed from 

the continuous record of instantaneous discharge and 

Qpub = the daily mean discharge previously published by 

the USGS. All days with disagreements greater than 5 

percent or less than -5 percent are either (1) days affected 

by ice, or (2) days in error in the published record. 

Disagreement increases slightly after the closure of Glen 

Canyon Dam because the increased variability in 

discharge within each day made it more difficult for the 

USGS to compute daily mean discharges prior to the 

availability of more advanced computers.

The Continuous Record of Instantaneous 

Discharge for Water Years 1921–2000

Following the computation and quality control of 

the continuous record of instantaneous discharge for water 

years 1921–86, these data were combined with 30- and 

15-minute stage and discharge data collected after 

October 1, 1986, to form a continuous record of the 

instantaneous discharge of the Colorado River at Lees 

Ferry from May 8, 1921, through September 30, 2000 

(fig. 21A). Also included in fig. 21A is the peak discharge 

of the 1884 flood, the largest flood between 1884 and 

1921 at Lees Ferry. Stage and discharge of the Colorado 

River at Lees Ferry were recorded at discrete 30-minute 

intervals between October 1, 1986, and May 31, 1998, and 

have been recorded at discrete 15-minute intervals since 

June 1, 1998. These post-October 1, 1986, data were 

previously and are currently available from the Arizona 

District of the USGS. The resolution of the May 8, 1921, 

through January 18, 1923, part of the continuous record 

(the period before installation of the Lees Ferry Gage with 

its continuous stage recorder) is twice daily; the resolution 

of the January 19, 1923, through September 30, 1986, part 

of the continuous record is approximately 15–30 minutes. 

The resolution of the October 1, 1986, through May 31, 

1998, part of the continuous record is 30 minutes; the 

resolution of the June 1, 1998, through September 30, 

2000, part of the continuous record is 15 minutes. The 

entire record can be either requested from the USGS 

Grand Canyon Monitoring and Research Center, 

Flagstaff, Arizona, or obtained electronically at 

http://www.gcmrc.gov. 

The daily range in the discharge of the 

Colorado River at Lees Ferry between May 8, 1921, and 

September 30, 2000, (fig. 21B) was computed from the 

data in fig. 21A. The largest daily range in discharge of 

68,100 ft3/s occurred on September 13, 1927, during the 

rising limb of the September 13, 1927, 125,000 ft3/s flood. 

Most of the water in this flood originated within the San 

Juan River Basin. 

100 Qcomp Qpub ( ) –

Qpub

-------------------------------------
40 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

ERROR, STAGE SHOULD BE

7.18 FEET, NOT 7.28 FEET.

midnight noon midnight midnight midnight noon noon

STAGE, IN FEET

NOVEMBER 1952

Figure 19. Example of the cause of an error in the published daily mean discharge record shown by the stage-recorder graph from November 10–12, 

1952. The daily mean stage for November 11 was 7.19 feet, but was incorrectly written on the graph as 7.29 feet. This mistake resulted in an error 

of -6.0 percent in the published daily mean discharge for November 11, 1952.
Computation of the Continuous Record of Instantaneous Discharge at Lees Ferry for Water Years 1921–2000 41

-100

-50

0

50

100

1-1-19301-1-19401-1-19501-1-19601-1-19701-1-1980

DISAGREEMENT BETWEEN THE PUBLISHED AND THE 

COMPUTED DAILY MEAN DISCHARGE TIME SERIES, IN PERCENT

CLOSURE OF 

GLEN CANYON DAM

RANGE OF PLUS OR MINUS 5-PERCENT DISAGREEMENT

DATE

Figure 20. Percent disagreement between the daily mean discharges computed from the continuous record of instantaneous discharge from this 

study and the published daily mean discharges. 
42 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

MAY 8, 1921–SEPTEMBER 30, 2000, CONTINUOUS RECORD OF INSTANTANEOUS DISCHARGE

PEAK DISCHARGE OF THE 1884 FLOOD NO FLOODS LARGER THAN THE 1921 FLOOD DURING THIS PERIOD

CLOSURE OF GLEN CANYON DAM

 DAM OPERATIONS

 CONSTRAINED

ON AUGUST 1, 1991

250,000

200,000

150,000

100,000

50,000

0

INSTANTANEOUS DISCHARGE, IN CUBIC FEET PER SECOND

1-1-18801-1-18901-1-19001-1-19101-1-19201-1-19301-1-19401-1-19501-1-19601-1-19701-1-19801-1-19901-1-2000

DATE

A.

Figure 21. (A) The continuous record of instantaneous discharge of the Colorado River at Lees Ferry, Arizona, May 8, 1921–September 30, 2000. 

ANALYSIS OF THE CONTINUOUS 

RECORD OF INSTANTANEOUS 

DISCHARGE

The final objectives of this study were to 

determine: (1) the pre-dam natural variability in the 

hydrology of the Upper Basin, (2) the effects of the 

operation of Glen Canyon Dam on the discharge of the 

Colorado River at Lees Ferry, and (3) the implications of 

the natural variability in hydrology and the effects of dam 

operations for sediment transport and storage in the reach 

downstream from Lees Ferry in Grand Canyon National 

Park. To accomplish these objectives, analyses of 

flow duration, sub-daily discharge variability, and 

flood frequency were conducted on the Lees Ferry 

instantaneous discharge data for May 8, 1921, through 

September 30, 2000, the available pre-gage historical 

flood data, and the 4,500-year paleoflood data of 

O’Connor and others (1994). 

Flow Duration, Sub-Daily Discharge 

Variability, and Sediment Transport during the 

Pre-Dam Period

To evaluate the natural variability in the discharge 

of the Colorado River at Lees Ferry over both long and 

short time scales and the implications of this natural 

discharge variability for sediment transport downstream 

from Lees Ferry, flow-duration analyses and analyses of 

the sub-daily variability in discharge were conducted on 

the pre-dam part of the continuous record of instantaneous 

discharge. These analyses indicate that, prior to the 

closure of Glen Canyon Dam, the duration of flows varied 

substantially over multi-year and decadal time scales. 

These analyses also indicate that the discharge of the predam river was fairly steady over sub-daily time scales, and 

that the daily range in discharge was the most extreme 

during the summer thunderstorm season.
Analysis of the Continuous Record of Instantaneous Discharge 43

B.

1-1-19201-1-19301-1-19401-1-19501-1-19601-1-19701-1-19801-1-19901-1-2000

DATE

10,000

20,000

30,000

40,000

50,000

60,000

70,000

DAILY RANGE IN DISCHARGE, IN CUBIC FEET PER SECOND

0

CLOSURE OF GLEN CANYON DAM

SEPTEMBER 13, 1927, FLOOD WITH

PEAK DISCHARGE OF 

125,000 CUBIC FEET PER SECOND

DAM OPERATIONS

CONSTRAINED 

ON AUGUST 1, 1991

Figure 21—Continued. (B) The daily range in the discharge of the Colorado River at Lees Ferry between May 8, 1921, and September 30, 2000.

The median discharge of the Colorado River 

at Lees Ferry during the entire pre-dam period was 

7,980 ft3/s (fig. 22A) and discharge varied considerably 

during each of the four pre-dam decades (fig. 22B). The 

decades with the highest discharges were the 1920s 

and 1940s, with median discharges of 10,700 ft3/s and 

8,460 ft3/s, respectively. The periods with the lowest 

discharges were the decade of the 1930s and the 12-year 

period between January 1, 1951, and March 12, 1963, 

with median discharges of 6,720 ft3/s and 7,210 ft3/s, 

respectively. The pre-dam month with the highest 

discharge was June, with a median discharge of 

51,200 ft3/s; the pre-dam month with the lowest discharge 

was January, with a median discharge of 5,140 ft3/s 

(fig. 23 and Appendix F). Operation of Glen Canyon Dam 

during 1963–2000 increased the median discharge of the 

Colorado River at Lees Ferry to 12,600 ft3/s (fig. 22A), a 

discharge 58 percent higher than the pre-dam median 

discharge and 18 percent higher than the median 

discharge during the wettest of the four pre-dam decades, 

the 1920s (fig. 22B). 

The decadal variation in the discharge of the 

Colorado River at Lees Ferry has major implications for 

the accumulation and storage of sediment in the reach in 

Grand Canyon National Park between the Lees Ferry and 

Grand Canyon gaging stations (that is, the reach in Marble 

and upper Grand Canyons). Topping and others (2000) 

showed that, as a likely result of the difference in 

hydraulic geometry between Glen Canyon and Marble 

Canyon, sand-transport rates at the Lees Ferry gaging 

station exceeded those at the Grand Canyon gaging station 

when the discharge was lower than about 9,000 ft3/s 

(Topping and others, 2000, fig. 4B). At discharges lower 

than about 7,000 ft3/s, more than an order of magnitude 

more sand was transported past the Lees Ferry gaging 

station than past the Grand Canyon gaging station, and at 
44 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

discharges lower than about 5,000 ft3/s, two orders of 

magnitude more sand was transported past the Lees Ferry 

gaging station than past the Grand Canyon gaging station. 

Therefore, when the discharge of the Colorado River was 

lower than about 9,000 ft3/s, sand accumulated in the 

reach in Marble and upper Grand Canyons. At discharges 

higher than about 9,000 ft3/s, however, sand-transport 

rates at the Grand Canyon gaging station generally 

equaled those at the Lees Ferry gaging station, and at 

discharges higher than about 16,000 ft3/s, sand-transport 

rates at the Grand Canyon gaging station generally either 

equaled or exceeded those at the Lees Ferry gaging 

station. Thus, at discharges greater than about 9,000 ft3/s, 

sand was either conveyed through or eroded from the 

reach in Marble and upper Grand Canyons. 

Of the four pre-dam decades, the 1920s were 

dominated by discharges that likely conveyed or eroded 

sand, the 1930s and 1950s were dominated by discharges 

that likely led to the accumulation of sand, and the 1940s 

were virtually neutral with respect to either accumulation 

or erosion of sand in Marble and upper Grand Canyons 

(fig. 22B). Discharges under which sand likely 

accumulated downstream from Lees Ferry were exceeded 

1920s (May 8, 1921–December 31, 1930) 

1930s (January 1, 1931–December 31, 1940) 

1940s (January 1, 1941–December 31, 1950) 

1950s–early 1960s (January 1, 1951–March 12, 1963) 

POST-DAM (March 14, 1963–September 30, 2000) 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

EXPLANATION

0 10 20 30 40 50 60 70 80 90 100

100

1,000

10,000

100,000

1,000,000

INSTANTANEOUS DISCHARGE, IN CUBIC FEET PER SECOND

100

1,000

10,000

100,000

1,000,000

INSTANTANEOUS DISCHARGE, IN CUBIC FEET PER SECOND

A.

B.

0 10 20 30 40 50 60 70 80 90 100

PRE-DAM (May 8–March 12, 1963)

POST-DAM (March 14, 1963–September 30, 2000) 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

EXPLANATION

SAND

CONVEYANCE

OR

EROSION

SAND

ACCUMULATION

PRE-DAM

 44.3%

POST-DAM

MEDIAN VALUE

 69.0%

SAND

CONVEYANCE

OR

EROSION

SAND

ACCUMULATION

POST-DAM

 69.0%

1920s

57.7%

1930s

36.5%

1940s

47.5%

1950s–early 1960s

 37.6%

MEDIAN VALUE

Figure 22. Flow-duration curves for the Colorado River at Lees Ferry. The 

gray shaded region shows the pre-dam discharge range under which sand 

accumulated in the reach between the Lees Ferry and Grand Canyon gaging 

stations, in Marble and upper Grand Canyons (Topping and others, 2000). The 

percentages of time when these discharges were exceeded during each 

period are indicated in italics. (A) Flow-duration curves for the pre- and postdam periods. (B) Flow-duration curves for the four pre-dam decades and the 

entire post-dam period. 

100

1,000

10,000

100,000

1,000,000

JAN. FEB. MAR. APR. MAY JUNE JULY AUG. SEPT. OCT. NOV. DEC.

PRE-DAM MONTHLY MEDIAN DISCHARGE

PRE-DAM MONTHLY MINIMUM DISCHARGE

PRE-DAM MONTHLY MAXIMUM DISCHARGE 

INSTANTANEOUS DISCHARGE, IN CUBIC FEET PER SECOND

EXPLANATION

MONTH

Figure 23. Pre-dam monthly median, minimum, and maximum discharges of 

the Colorado River at Lees Ferry.
Analysis of the Continuous Record of Instantaneous Discharge 45

57.7 percent of the time during the 1920s, 36.5 percent of 

the time during the 1930s, 47.5 percent of the time during 

the 1940s, and 36.6 percent of the time during the 12-year 

period of the 1950s–early 1960s. During the entire predam period, these discharges were exceeded only 44.3 

percent of the time. Likewise, based on discharge alone, 

the pre-dam months of January through March and 

August through December would be characterized by the 

accumulation of sand, and April through July would be 

characterized by the erosion of sand (fig. 23 and Appendix 

F). Because of the greater tributary supply of sand during 

July (Topping and others, 2000, fig. 10A), however, sand 

also accumulated in Marble and upper Grand Canyons 

during this month (Topping and others, 2000, fig. 10C). 

Discharge of the pre-dam river at Lees Ferry was 

fairly steady. Therefore, the difference between flowduration curves computed from either the continuous 

record of instantaneous discharge from this study or the 

published record of daily mean discharge is minimal 

(fig. 24). The pre-dam median daily range in discharge 

was only 542 ft3/s (fig. 25A). Over decadal time scales, 

the daily range in discharge of the pre-dam river was 

somewhat correlated with discharge (fig. 25B). The two 

wettest decades, the 1920s and 1940s, had the largest 

median daily ranges in discharge, 808 and 566 ft3/s, 

respectively. The 1930s were the driest decade with the 

lowest discharges, but had the second smallest median 

daily range in discharge, 516 ft3/s. The 12-year period 

from January 1, 1951, through March 12, 1963, was 

slightly wetter than the 1930s, but had the smallest 

median daily range in discharge, 416 ft3/s. The month 

with the greatest median daily range in discharge was 

June during the snowmelt flood (fig. 26 and Appendix G), 

although the daily ranges in discharge were most extreme 

during the summer thunderstorm season of July through 

October (fig. 26). The pre-dam daily range in discharge 

was largest on September 13, 1927, when discharge 

increased by 68,100 ft3/s at Lees Ferry to a peak discharge 

of 125,000 ft3/s as the result of a flood that mostly 

originated within the San Juan River drainage basin 

(fig. 21B). Although such extreme examples exist, large 

daily ranges in discharge were rare during the pre-dam 

era, with only 1 percent of all days having a daily 

discharge range in excess of 10,000 ft3/s (fig. 27). Thus, 

the median daily ranges in discharge during the months 

of July through October were actually lower than those 

during the snowmelt flood months of April through June 

(fig. 26).

Effects of the Operation of Glen Canyon 

Dam on Flow Duration, Sub-Daily Discharge

Variability, and Sediment Transport

To determine the effects of the operation of Glen 

Canyon Dam on the hydrology of the Colorado River at 

Lees Ferry and on sediment transport in the Colorado 

River downstream from Lees Ferry, analyses of flow 

duration and sub-daily variability in discharge were also 

conducted on the post-dam part of the continuous record 

of instantaneous discharge. As shown by these analyses, 

not only has operation of the dam reduced the duration of 

flood flows, it has greatly reduced the duration of low 

flows shown by Topping and others (2000) to be 

important for the accumulation and storage of sand in the 

Colorado River between the Lees Ferry and Grand 

Canyon gaging stations (fig. 22A). Prior to the closure of 

Glen Canyon Dam in 1963, the natural discharge of the 

Colorado River at Lees Ferry was lower than 7,980 ft3/s 

half of the time. By the 1990s, operation of the dam had 

increased the base flows in the river such that the 

discharge at Lees Ferry was lower than 8,000 ft3/s only 7 

percent of the time. In addition to radically changing the 

hydrology, operation of the dam for power generation has 

introduced daily fluctuations in discharge that are much 

larger and more common than those that generally 

occurred prior to closure of the dam (figs. 21B and 25A). 

0 10 20 30 40 50 60 70 80 90 100 100

1,000

10,000

100,000

1,000,000

PRE-DAM FLOW-DURATION CURVE COMPUTED 

FROM INSTANTANEOUS DISCHARGES

PRE-DAM FLOW-DURATION CURVE COMPUTED 

FROM DAILY MEAN DISCHARGES 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

DISCHARGE, IN CUBIC FEET PER SECOND

Figure 24. Comparison of the pre-dam flow-duration curve computed from 

instantaneous discharges with the pre-dam flow-duration curve computed 

from daily mean discharges. 
46 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

0 10 20 30 40 50 60 70 80 90 100

10

100

1,000

10,000

100,000

PRE-DAM 

(May 8, 1921–March 12, 1963)

POST-DAM 

(March 14, 1963–September 30, 2000) 

MEDIAN VALUE

0 10 20 30 40 50 60 70 80 90 100

10

100

1,000

10,000

100,000

1920s 

(May 8, 1921–December 31, 1930) 

1930s 

(January 1, 1931–December 31, 1940) 

1940s 

(January 1, 1941–December 31, 1950) 

1950s–early 1960s 

(January 1, 1951–March 12, 1963) 

POST-DAM 

(March 14, 1963–September 30, 2000) 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

DAILY RANGE IN DISCHARGE, IN CUBIC FEET PER SECOND

EXPLANATION

EXPLANATION

MEDIAN VALUE

A.

B.

Figure 25. Exceedance curves of the daily range in the discharge of the Colorado River at Lees Ferry: (A) For the pre- and post-dam periods. 

(B) For the four pre-dam decades and the entire post-dam period.

Operation of Glen Canyon Dam has evolved over 

the four decades since closure of the dam on March 13, 

1963 (fig. 21A). From March 1963 through the latter part 

of 1964, releases from Glen Canyon Dam were kept low 

(with discharges kept close to 1,000 ft3/s for most of this 

period) to increase the storage of water in Lake Powell 

reservoir quickly. Then, from April through June 1965, a 

series of artificial floods was released from the dam, with 

six of these floods having peak discharges greater than 

50,000 ft3/s. Between these flood peaks, discharges were 

reduced to as little as 10,000 ft3/s. These 1965 floods were 

designed to raise the elevation of Lake Mead reservoir and 

to scour the reach immediately below Glen Canyon Dam 

in order to increase the efficiency of the power plant at the 
Analysis of the Continuous Record of Instantaneous Discharge 47

dam, and were referred to informally by Bureau of 

Reclamation engineers as "channel cleaning flows" 

(Grams and others, Utah State University, written 

commun., 2002). During these 3 months of high 

discharge, approximately 5.0 million tons of fine sediment 

(that is, sand and finer material) were scoured from Glen 

Canyon between the dam and Lees Ferry (computed on 

the basis of the USGS daily suspended-sediment data 

from the Lees Ferry gaging station; U.S. Geological 

Survey, accessed November 15, 2000), and approximately 

17.6 million tons of fine sediment were scoured from the 

reach between the Lees Ferry and Grand Canyon gaging 

stations (Rubin and Topping, 2001). From July 1965 to 

June 6, 1980, the dam was operated in such a way 

as to fill Lake Powell reservoir gradually, to fulfill 

downstream water-delivery obligations, and to maximize 

the generation of hydroelectric power. As shown in this 

study, this "reservoir-filling period" was the period 

of the greatest daily fluctuations in discharge for power 

generation (fig. 28). From the initial filling of the reservoir 

on June 6, 1980, until August 1, 1991, the dam was 

operated as it was prior to 1980, with the additional safety 

constraint of drawing down the reservoir early in the 

year to provide enough volume for the snowmelt flood. 

This period included the largest post-dam flood, the 

97,000 ft3/s June 1983 flood (Martin, 1989). After this 

flood, dam operations were revised to release more water 

earlier in the spring to reduce the frequency of spills of 

this magnitude (National Research Council, 1996). Since 

August 1, 1991, dam releases have been constrained in an 

attempt to minimize the downstream effects of the dam on 

the Colorado River ecosystem in Glen Canyon National 

Recreation Area and Grand Canyon National Park 

(National Research Council, 1996). Under these 

constraints, (1) discharges no lower than 5,000 ft3/s could 

be released, (2) discharges no lower than 8,000 ft3/s could 

be released during the day, (3) discharges no greater than 

20,000 ft3/s could be released (this upper limit was 

increased to 25,000 ft3/s under the 1996 Record of 

Decision signed by the Secretary of the Interior), and 

(4) additional limits were placed on the daily range in 

discharge and the rates at which discharge could change 

(ramping rates). 

1

10

100

1,000

10,000

100,000

JAN. FEB. MAR. APR. MAY JUNE JULY AUG. SEPT. OCT. NOV. DEC.

PRE-DAM MONTHLY MEDIAN DAILY RANGE IN DISCHARGE

PRE-DAM MONTHLY MINIMUM DAILY RANGE IN DISCHARGE

PRE-DAM MONTHLY MAXIMUM DAILY RANGE IN DISCHARGE

DAILY RANGE IN DISCHARGE, IN CUBIC FEET PER SECOND

EXPLANATION

MONTH

Figure 26. Pre-dam monthly median, minimum, and maximum daily ranges in 

the discharge of the Colorado River at Lees Ferry. Daily ranges in discharge 

plotted equal to 1 indicate that the minimum daily range in discharge during 

these months was less than the level of detection. 

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

0.001

0.01

0.1

15

102030507080909599

99.9

99.99

99.999

PRE-DAM ALL (May 9, 1921–March 12, 1963)

PRE-DAM NOVEMBER THROUGH JUNE

PRE-DAM JULY THROUGH OCTOBER (summer thunderstorm season) 

DAILY RANGE IN DISCHARGE, IN CUBIC FEET PER SECOND

PERCENTAGE OF DAYS WITH RANGE IN DISCHARGE 

LESS THAN VALUE SHOWN

EXPLANATION

Figure 27. Probability graph of the pre-dam daily range in the discharge of the 

Colorado River at Lees Ferry. Daily ranges in discharge prior to the closure of 

Glen Canyon Dam were the most extreme during the summer thunderstorm 

season (July through October).
48 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

As a result of the changing operation of Glen 

Canyon Dam through time, the discharge characteristics 

of the Colorado River at Lees Ferry have changed 

substantially during the four decades since closure of the 

dam; the median discharge has generally increased, and 

the duration of lower discharges has progressively 

decreased (fig. 29). Thus, during each decade, as 

discharges less than 9,000 ft3/s became less common, the 

discharge of the river became progressively more 

conducive to the conveyance of sand through or the 

erosion of sand from Marble and upper Grand Canyons. 

The median discharge for the entire post-dam period of 

March 14, 1963, through September 30, 2000, was 

12,600 ft3/s. During the initial reservoir-filling period of 

March 14, 1963, through December 31, 1970, the 

median discharge was 9,490 ft3/s, and the discharge 

of the river was greater than 9,000 ft3/s 52.7 percent of 

the time. During the 1970s, the median discharge was 

11,600 ft3/s, and the discharge of the river was greater 

than 9,000 ft3/s 62.2 percent of the time. During the 

1980s, the combination of the wet-weather conditions in 

the Upper Basin and the full reservoir resulted in the 

highest decadal median discharge, 15,900 ft3/s. Discharge 

of the river during the 1980s was greater than 9,000 ft3/s 

75.5 percent of the time. Due to the constraints placed on 

dam operations in 1991, the 1990s were the post-dam 

decade with the most discharges conducive to the

conveyance of sand through or the erosion of sand from 

Marble and upper Grand Canyons. Although the median 

discharge during the 1990s (13,500 ft3/s) was less than 

that during the 1980s, the 1990s had the longest duration 

of discharges greater than 9,000 ft3/s (82.6 percent of the 

time). 

In addition to greatly reducing the duration of 

flood flows and progressively eliminating lower flows, 

operation of the dam has eliminated the seasonality 

from the annual hydrograph of the Colorado River 

(fig. 30). Prior to closure of the dam, the lowest discharge 

month was January, with a median discharge of 5,140 ft3/s 

(fig. 30A) and a minimum discharge of 578 ft3/s (fig. 

30B), and the highest discharge month was June, with a 

median discharge of 51,200 ft3/s (fig. 30A) and a 

maximum discharge of 170,000 ft3/s (fig. 30B). During 

the post-dam period of record, the highest-discharge 

month was August, with a median discharge of 

16,400 ft3/s, and the lowest discharge month was October, 

with a median discharge of 10,200 ft3/s (fig. 30A). 

0

5,000

10,000

15,000

20,000

25,000

30,000

7-8-19777-9-1977

7-10-19777-11-19777-12-19777-13-19777-14-1977

INSTANTANEOUS DISCHARGE, IN CUBIC FEET PER SECOND

DATE

Figure 28. An example of large dam-induced daily fluctuations in the 

discharge of the Colorado River at Lees Ferry.

0 10 20 30 40 50 60 70 80 90 10 100

1,000

10,000

100,000

1,000,000

PERCENTAGE OF TIME EQUALED OR EXCEEDED

INSTANTANEOUS DISCHARGE, IN CUBIC FEET PER SECOND

SAND

CONVEYANCE

OR

EROSION

SAND

ACCUMULATION

PRE-DAM

 44.3%

1960s

52.7%

1980s

75.5%

1970s

62.2%

1990s

82.6%

EXPLANATION MEDIAN VALUE

1960s (March 14, 1963–December 31, 1970)

1970s (January 1, 1971–December 31, 1980)

1980s (January 1, 1981–December 31, 1990)

1990s (January 1, 1991–September 30, 2000)

PRE-DAM (May 8, 1921–March 12, 1963) 

Figure 29. Flow-duration curves for the Colorado River at Lees Ferry from each 

of the four post-dam decades and the entire pre-dam period. The gray shaded 

region shows the pre-dam discharge range under which sand accumulated in 

the reach between the Lees Ferry and Grand Canyon gaging stations, in Marble 

and upper Grand Canyons. The percentages of time when these discharges 

were exceeded during each period are indicated in italics. 
Analysis of the Continuous Record of Instantaneous Discharge 49

100

1,000

10,000

100,000

1,000,000

PRE-DAM 

POST-DAM 

MONTHLY MEDIAN DISCHARGE

A.

B.

100

1,000

10,000

100,000

1,000,000

JAN. FEB. MAR. APR. MAYJUNEJULY AUG.SEPT.OCT.NOV.DEC.

PRE-DAM 

POST-DAM 

PRE-DAM 

POST-DAM 

INSTANTANEOUS DISCHARGE, IN CUBIC FEET PER SECOND

MONTHLY MINIMUM DISCHARGE

MONTHLY MAXIMUM DISCHARGE

MONTH

JAN. FEB. MAR. APR. MAYJUNEJULY AUG.SEPT.OCT.NOV.DEC.

Figure 30. Effects of dam operations on: (A) Monthly median discharges of the Colorado River at Lees 

Ferry. (B) Monthly minimum and maximum discharges of the Colorado River at Lees Ferry. All of the postdam monthly minimum discharges in this figure occurred prior to the constraints placed on dam 

operations on August 1, 1991.
50 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

Thus, operation of the dam has reduced the natural range 

in the monthly median discharge by a factor of eight. 

Furthermore, because no post-dam months have median 

discharges less than 9,000 ft3/s, operation of the dam has 

biased all months of the year toward either the conveyance 

of sand through or the erosion of sand from Marble and 

upper Grand Canyons.

Operation of Glen Canyon Dam for hydroelectricpower generation has increased the daily range in 

discharge by a factor of 15.8, with the post-dam median 

daily range in discharge, 8,580 ft3/s, actually exceeding 

the pre-dam median discharge, 7,980 ft3/s (figs. 22A and 

25A). In addition, dam operations have increased the 

median daily range in discharge by a factor of 10.6 over 

the pre-dam decade with the largest daily range in 

discharge, the 1920s (fig. 25B). Because of this increased 

sub-daily variability in discharge, daily mean discharges 

are not adequate to characterize the hydrology of the postdam river. Thus, the post-dam flow-duration curve 

computed from daily mean discharges underestimates 

the duration of higher discharges and overestimates the 

duration of lower discharges (fig. 31). Of the four postdam decades, the 1970s had the greatest median daily 

range in discharge, 13,700 ft3/s, and the 1990s had the 

smallest median daily range in discharge, 4,940 ft3/s 

(fig. 32). The median daily ranges in discharge during the 

1960s and 1980s were intermediate to these values at 

8,840 ft3/s and 9,830 ft3/s, respectively. For comparison, 

the pre-dam median daily range in discharge was 542 ft3/s 

(fig. 32). As in the case of discharge, operation of Glen 

Canyon Dam has also eliminated the seasonality 

associated with the daily range in discharge (fig. 33). 

Operation of the dam for hydroelectric-power generation 

has increased the daily range in discharge during all but 

0.1 percent of all days (fig. 34).

Not only has operation of the dam greatly reduced 

the duration of lower discharges, it has eliminated lowdischarge months (fig. 35). Prior to closure of the dam, 

minimum discharges could be lower than 5,000 ft3/s each 

month of the year (figs. 30A and 35A), and maximum 

discharges could be lower than 9,000–10,000 ft3/s in 

several or more months each year (fig. 35B). After 1965, 

discharges released from the dam were not held lower 

than 9,000 ft3/s for more than 1 month until the summer of 

2000, when the discharge released from the dam was held 

steady at 8,000 ft3/s from June through early September. 

0 10 20 30 40 50 60 70 80 90 100 100

1,000

10,000

100,000

1,000,000

POST-DAM FLOW-DURATION CURVE COMPUTED 

FROM INSTANTANEOUS DISCHARGES

POST-DAM FLOW-DURATION CURVE COMPUTED 

FROM DAILY MEAN DISCHARGES 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

DISCHARGE, IN CUBIC FEET PER SECOND

Figure 31. Comparison of the post-dam flow-duration curve computed from 

instantaneous discharges with the post-dam flow-duration curve computed 

from daily mean discharges.

0 10 20 30 40 50 60 70 80 90 100 10

100

1,000

10,000

100,000

1960s (March 14, 1963–December 31, 1970)

1970s (January 1, 1971–December 31, 1980) 

1980s (January 1, 1981–December 31, 1990)

1990s (January 1, 1991–September 30, 2000) 

PRE-DAM (May 8, 1921–March 12, 1963) 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

DAILY RANGE IN DISCHARGE, IN CUBIC FEET PER SECOND

EXPLANATION MEDIAN VALUE

Figure 32. Exceedance curves of the daily range in the discharge of the 

Colorado River at Lees Ferry for the four post-dam decades and the entire 

pre-dam period of record. 
Analysis of the Continuous Record of Instantaneous Discharge 51

Although the post-dam duration of low discharges decreased 

after closure of the dam (figs. 22A and 29), monthly minimum 

discharges during the post-dam period prior to August 1, 

1991, were actually lower than the monthly minimum 

discharges during the pre-dam months of February through 

1

10

100

1,000

10,000

100,000

PRE-DAM

POST-DAM

DAILY RANGE IN DISCHARGE, IN CUBIC FEET PER SECOND

MONTHLY MEDIAN DAILY RANGE IN DISCHARGE

1

10

100

1,000

10,000

100,000

JAN. FEB. MAR. APR. MAY JUNE JULY AUG. SEPT. OCT. NOV. DEC.

JAN. FEB. MAR. APR. MAY JUNE JULY AUG. SEPT. OCT. NOV. DEC.

PRE-DAM POST-DAM

PRE-DAM POST-DAM

MONTHLY MINIMUM DAILY RANGE IN DISCHARGE

MONTHLY MAXIMUM DAILY RANGE IN DISCHARGE

MONTH

A.

B.

Figure 33. The effects of dam operations on: (A) Monthly median daily ranges in 

the discharge of the Colorado River at Lees Ferry. (B) Monthly minimum and 

maximum daily ranges in the discharge of the Colorado River at Lees Ferry. Daily 

ranges in discharge plotted equal to 1 indicate that the minimum daily range in 

discharge during these months was less than the level of detection.

November (figs. 30B and 35A). After August 1, 1991, 

discharges lower than 5,000 ft3/s could not be released 

from the dam at any time (fig. 35A) and discharges 

lower than 8,000 ft3/s could be released from the dam 

only at night (U.S. Department of the Interior, 1995; 

National Research Council, 1996). Because of the 

slightly wetter weather patterns in the Upper Basin 

during the mid- to late-1990s, a full reservoir, and these 

new constraints on dam operations, discharges lower 

than the pre-dam median discharge were no longer a 

substantial part of the hydrology of the Colorado River 

downstream from Glen Canyon Dam by the late 1990s. 

Likewise, by the late 1990s, discharges lower than 

the maximum pre-dam discharge at which sand 

accumulated in Marble and upper Grand Canyons 

(9,000 ft3/s) were no longer a substantial part of the 

hydrology of the Colorado River downstream from 

Glen Canyon Dam. 

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000 0.001

0.01

0.1

1

5

10

20

3050

70

80

90

95

99

99.9

99.99

99.999

PRE-DAM 

(May 9, 1921–March 12, 1963)

POST-DAM 

(March 14, 1963–September 30, 2000) 

DAILY RANGE IN DISCHARGE, IN CUBIC FEET PER SECOND

PERCENTAGE OF DAYS WITH RANGE IN DISCHARGE

LESS THAN VALUE SHOWN

Figure 34. Probability graph of the daily range in the discharge of the 

Colorado River at Lees Ferry during the pre- and post-dam periods of record. 

Operation of Glen Canyon Dam has greatly increased the daily range in 

discharge during most days.
52 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

Flood Frequency During the 

Pre-Dam Period

To evaluate the natural frequency of floods on the 

Colorado River at Lees Ferry, partial-duration and annual 

flood-frequency analyses were conducted on the pre-dam 

part of the continuous record of instantaneous discharge 

(fig. 36). To extend these analyses to larger floods with 

longer return periods, the 1884 flood and the paleoflood 

data of O’Connor and others (1994) were also included 

in these analyses. The base discharge selected for the 

partial-duration analysis was the instantaneous discharge 

between May 8, 1921, and March 12, 1963, that 

was equaled or exceeded only 25 percent of the time, 

18,500 ft3/s (fig. 22A). This base discharge was also 

chosen because it was exceeded about 25 percent of the 

time during both the pre- and post-dam periods of record. 

In this section of the paper, the term "flood" is used to 

describe any discharge above this base discharge. During 

the pre-dam period between May 8, 1921, and March 12, 

1963, 277 floods occurred with peak discharges in excess 

of 18,500 ft3/s. The return periods for the pre-dam flood 

data were computed on the basis of different lengths of 

record. The return periods for the 277 floods between 

May 8, 1921, and March 12, 1963, were computed on the 

basis of their 41.8-year period of record. Because the 1884 

flood had the largest peak discharge of any flood at Lees 

Ferry between June 1884 and March 1963, the return 

interval for the 1884 flood was computed on the basis of a 

78.2-year period of record. The return period of the largest 

paleoflood identified by O’Connor and others (1994), the 

~300,000 ft3/s paleoflood that left behind the crevice 

deposit, was recomputed on the basis of the 1,200–

1,600-year age of the deposit rather than the 2,307-year 

period of record used by O’Connor and others (1994). 

Inclusion of the data of O’Connor and others 

(1994) in this flood-frequency analysis first required a 

reevaluation of the return periods for these paleofloods. 

O’Connor and others assumed that the 10 paleofloods 

which left the deposits in Axehandle Alcove that are 

less than 2,062–2,307 years old (including the historic 

flood that produced deposit "G1") were equally spaced 

over 2,307 years. This assumption led O’Connor and 

others to compute return periods of 200 to 800 years 

for the eight paleofloods associated with the deposits 

underlying "G1." During the early part of the 20th 

century, however, floods observed at the Yuma 

gaging station on the Colorado River had peak 

discharges equivalent to those of these eight prehistoric 

paleofloods. Because the natural June tributary 

inflow to the Colorado River between Lees Ferry and 

Yuma was minimal, the peak discharges of these floods at 

Lees Ferry and Yuma should have been comparable. 

Between 1903 and 1920, five snowmelt floods at the 

Yuma gaging station had peak discharges that ranged from 

either 120,000 to 170,000 ft3/s (when a coefficient of 

0.8 is used to relate the measured surface velocities to

the mean velocities, as explained previously) or 135,000 

to 190,000 ft3/s (when a coefficient of 0.9 is used). 

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

1-1-19201-1-19301-1-19401-1-19501-1-19601-1-19701-1-19801-1-19901-1-2000

CLOSURE OF GLEN CANYON DAM

DAM OPERATIONS

CONSTRAINED

ON AUGUST 1, 1991

0

50,000

100,000

150,000

200,000

1-1-19201-1-19301-1-19401-1-19501-1-19601-1-19701-1-19801-1-19901-1-2000

MAXIMUM DISCHARGE EACH MONTH, IN CUBIC FEET PER SECOND MINIMUM DISCHARGE EACH MONTH, IN CUBIC FEET PER SECOND

CLOSURE OF GLEN CANYON DAM

DAM OPERATIONS

CONSTRAINED

ON AUGUST 1, 1991

DATE

A.

B.

Figure 35. The minimum and maximum discharges of the Colorado River 

at Lees Ferry each month from May 1921–September 2000. (A) Minimum 

discharge each month. (B) Maximum discharge each month. 
Analysis of the Continuous Record of Instantaneous Discharge 53

10,000

100,000

0.001 0.01 0.1 1 10 100 1,000 10,000

PEAK DISCHARGE, IN CUBIC FEET PER SECOND

RETURN PERIOD, IN YEARS

"G1" 1921 FLOOD

1884 FLOOD

500,000

50,000

200,000

20,000

8 DEPOSITS

UNDERLYING 

DEPOSIT "G1"

1

2

PRE-DAM PARTIAL-DURATION FLOOD SERIES FROM LEES FERRY GAGING STATION RECORD

PRE-DAM ANNUAL FLOOD SERIES FROM LEES FERRY GAGING STATION RECORD 

PALEOFLOOD ASSOCIATED WITH THE 1,200–1,600-YEAR-OLD CREVICE DEPOSIT

PALEOFLOOD DATA COMPUTED FROM THE REVISED DISCHARGES FROM THIS STUDY 

AND THE ORIGINAL RETURN PERIODS FROM O'CONNOR AND OTHERS (1994) 

UPWARD EXTENSION OF PRE-DAM PARTIAL-DURATION FLOOD SERIES BASED 

ON THE PALEOFLOOD DATA COMPUTED FROM THE REVISED DISCHARGES 

AND ADJUSTED RETURN PERIODS FROM THIS STUDY 

EXPLANATION

Figure 36. Partial-duration and annual flood-frequency analyses for the pre-dam Colorado River at Lees Ferry. In the partial-duration series, 40 

out of the 46 floods with peak discharges in excess of 50,000 cubic feet per second were snowmelt floods. Also included are the paleoflood 

data of O’Connor and others (1994) recomputed on the basis of the revised discharges from this study. The bold black arrows show the returnperiod adjustments that should be applied to paleoflood data. Arrow "1" indicates the return-period adjustment that should be applied to the 

paleoflood associated with deposit "G1" (given that "G1" was likely deposited during the 1921 flood), and arrow "2" indicates the return-period 

adjustments that should be applied to the paleofloods associated with the eight deposits underlying "G1" (see text for further discussion). 
54 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

The peak discharges of these five floods at Lees Ferry, 

therefore, were smaller or equal to the peak discharge of 

the 1921 flood that likely produced the "G1" deposit, 

larger than the peak discharges of all post-1921 floods, 

and were in the same discharge range as the eight 

paleofloods that left the deposits underlying deposit "G1." 

The fact that the eight prehistoric paleofloods 

and the large 1903–20 floods at the Yuma gaging station 

were of comparable magnitudes requires that floods of 

these magnitudes occurred at least twice in the last 200–

800 years. Thus, the return-period estimates of O’Connor 

and others (1994) for the paleofloods are high by a 

minimum of a factor of two (herein referred to as 

bounding condition one). Extension of the record of 

floods at the Lees Ferry gaging station from 1921 back to 

1903 through use of the record from the Yuma gaging 

station, yields return periods for these floods that range 

from 12 to 60 years (herein referred to as bounding 

condition two). Therefore, on the basis of these two 

bounding conditions, the return period for the smallest of 

the eight paleofloods must range from 12 to 100 years 

over the last 2,307 years, and the return period for the 

largest of the eight paleofloods must range from 60 to 

400 years over the last 2,307 years. Furthermore, the 

fact that the 1884 and 1921 floods were larger than the 

eight paleofloods suggests that the return periods for the 

eight paleofloods are closer to those computed on the 

basis of bounding condition two (12 to 60 years). The 

return-period adjustments that should be applied to the 

paleoflood data based on this analysis are indicated in 

fig. 36. Finally, although the paleoflood discharges used 

by O’Connor and others (1994) were too high and the 

return periods computed by O’Connor and others were 

too long, the upward extension of the flood-frequency 

relationship in fig. 36 is quite similar to that originally 

proposed in fig. 5 of O’Connor and others (1994); in both 

analyses, a flood with a peak discharge of 300,000 ft3/s 

has a return period slightly longer than 1,000 years. 

Comparison of the flood-frequency analyses based 

on the partial-duration and annual flood series indicates 

that the analysis based on the annual flood series 

overestimates the return periods for all pre-dam floods 

with peak discharges lower than about 60,000 ft3/s 

(fig. 36). The partial-duration flood-frequency analysis 

indicates that, prior to the closure of Glen Canyon Dam, 

floods with a peak discharge of about 50,000 ft3/s 

occurred every year on average, and that floods with a 

peak discharge of about 125,000 ft3/s occurred every 

8 years on average. This analysis also suggests that, under 

natural pre-dam hydrology, the return period for the 

170,000 ft3/s 1921 flood was about 40 years, the return 

period for the 210,000 ft3/s 1884 flood was about 80 

years, and that the return period of a 300,000 ft3/s flood 

was slightly longer than 1,000 years (fig. 36).

Effect of the Operation of Glen Canyon 

Dam on Flood Frequency

To determine the effect of the operation of Glen 

Canyon Dam on the frequency of floods on the Colorado 

River, partial-duration and annual flood-frequency 

analyses were also conducted on the post-dam part of the 

continuous record of instantaneous discharge (fig. 37A). 

The same base discharge was used in the post-dam partialduration flood-frequency analysis as in the pre-dam 

analysis (18,500 ft3/s). During the post-dam period 

between March 14, 1963, and September 30, 2000, 5,222 

floods had peak discharges in excess of 18,500 ft3/s. The 

return periods for these 5,222 post-dam floods were 

computed on the basis of a 37.6-year period of record. 

Because operation of Glen Canyon Dam has effectively 

removed the seasonality from the annual hydrograph of 

the Colorado River (fig. 30), flood-frequency analysis 

based on the annual flood series provides a poor 

characterization of the frequency of floods in the post-dam 

river (fig. 37A). Compared to the return periods computed 

from the partial-duration flood-frequency analysis, the 

annual flood-frequency analysis greatly overestimates the 

return period of all but the three largest post-dam floods. 

The effect of the operation of Glen Canyon Dam 

on the frequency of floods on the Colorado River at Lees 

Ferry is shown in fig. 37B. Operation of the dam has 

increased the return period for floods with pre-dam return 

periods in excess of about 6–7 months, and has decreased 

the return periods for the smallest floods in the analysis 

(floods with peak discharges of about 18,500 ft3/s) from 

1.8 months to only 2.6 days. The effect of dam operations 

on the duration of floods with sustained discharge in 

excess of 18,500 ft3/s has also been large (fig. 38). 

The longest four periods of sustained discharge above 

18,500 ft3/s at Lees Ferry have all occurred since the dam 

was closed. In 1984, discharge exceeded 18,500 ft3/s for 

214.2 consecutive days; in 1997, discharge exceeded 

18,500 ft3/s for 200.7 consecutive days; in 1983, discharge 

exceeded 18,500 ft3/s for 175.0 consecutive days; 

and in 1985, discharge exceeded 18,500 ft3/s for 138.4 

consecutive days. The longest pre-dam period of sustained 

higher discharge occurred in 1929, when discharge 

exceeded 18,500 ft3/s for 129.6 consecutive days. 
Analysis of the Continuous Record of Instantaneous Discharge 55

10,000

100,000

0.001 0.01 0.1 1 10 100 1,000 10,000

POST-DAM PARTIAL-DURATION FLOOD SERIES 

POST-DAM ANNUAL FLOOD SERIES 

500,000

50,000

200,000

20,000

10,000

100,000

0.001 0.01 0.1 1 10 100 1,000 10,000

PRE-DAM PARTIAL-DURATION 

FLOOD SERIES 

POST-DAM PARTIAL-DURATION 

FLOOD SERIES 

PEAK DISCHARGE, IN CUBIC FEET PER SECOND

RETURN PERIOD, IN YEARS

500,000

50,000

200,000

20,000

EXPLANATION

A.

B.

UPWARD EXTENSION OF

PRE-DAM PARTIALDURATION FLOOD SERIES

BASED ON THE

PALEOFLOOD DATA

COMPUTED FROM THE 

REVISED DISCHARGES

AND ADJUSTED 

RETURN PERIODS

FROM THIS STUDY

Figure 37. The effect of dam operations on flood frequency on the Colorado River at Lees Ferry. (A) Comparison of post-dam partial-duration and annual 

flood-frequency analyses. The annual flood-frequency analysis grossly overestimates the return periods for most post-dam floods. (B) Partial-duration 

flood-frequency analyses for the pre- and post-dam period. 
56 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

0

50

100

150

200

250

1-1-19201-1-19301-1-19401-1-19501-1-19601-1-19701-1-19801-1-19901-1-2000

DURATION OF SUSTAINED DISCHARGE OVER 

18,500 CUBIC FEET PER SECOND, IN DAYS

1983

1984

1985

1997

CLOSURE OF 

GLEN CANYON DAM

1929

DATE

Figure 38. The duration of periods of sustained discharge over 18,500 cubic feet per second (the base discharge used in the flood-frequency analyses). 

CONCLUSIONS

Although its geographic location within the 

Colorado River drainage basin and its accessibility made 

Lees Ferry a good location for a stream-gaging station, 

the stage dependence of the location and geometry of the 

hydraulic control made computations of flood discharges 

at this gaging station difficult during the pre-dam period. 

As stage increases, the water-surface profile progressively 

flattens at the Lees Ferry Gage as the location of hydraulic 

control shifts downstream. This development of 

backwatered flow conditions causes a reversal in the 

curvature of the stage-discharge rating curve, such that, 

at stages in excess of about 15 ft, discharge is proportional 

to stage raised to a power of less than one. This reversal 

in curvature made extrapolation of the stage-discharge 

rating curve problematic; thus there was considerable 

uncertainty in the estimated peak discharges of the largest 

historical floods at Lees Ferry in 1884 and 1921. The 

USGS originally estimated that these two floods had peak 

discharges of about 210,000–250,000 and 174,000 ft3/s, 

respectively. On the basis of a key assumption now known 
Conclusions 57

to be false, the peak discharges of these floods were 

revised upward by Gatewood and Hunter in 1938 to 

300,000 and 220,000 ft3/s, respectively. The analyses 

in this paper of the raw data from Lees Ferry (which 

accounts for the progressive development of backwatered 

flow conditions in the reach at high stages), of the surveys 

of high-water marks at the Grand Canyon gaging station 

and at River-mile 233.7, and of the published data from 

other upstream and downstream gaging stations, indicate 

that the most likely peak discharges of the 1884 and 1921 

floods at Lees Ferry were 210,000±30,000 ft3/s and 

170,000±20,000 ft3/s, values close to those originally 

estimated by the USGS. 

The progressive flattening of the water-surface 

profile at high stages and the consequent downward 

revision in the peak discharges of these two floods imply 

that the peak discharges in the 4,500-year paleoflood 

series of O’Connor and others (1994) need to be revised. 

The peak discharges of many of the prehistoric floods 

associated with these paleoflood deposits were, in fact, 

comparable to the peak discharges of floods during the 

early part of the 20th century. This observation leads to a 

minimum reduction of a factor of two in the return periods 

for these floods estimated by O’Connor and others (1994). 

Analyses of the 1921–2000 continuous record of 

instantaneous discharge provide insight into the pre-dam 

natural variability in the discharge of the Colorado River, 

and the hydrologic changes imposed on the Colorado 

River in Grand Canyon National Park by the construction 

and operation of Glen Canyon Dam. The continuous 

record of instantaneous discharge can be either requested 

from the USGS Grand Canyon Monitoring and Research 

Center, Flagstaff, Arizona, or obtained electronically at 

http://www.gcmrc.gov. 

Prior to the closure of Glen Canyon Dam in March 

1963, the discharge of the Colorado River at Lees Ferry 

varied substantially over decadal time scales. The median 

discharge during the driest decade, the 1930s (6,720 ft3/s), 

differed from the median discharge during the wettest 

decade, the 1920s (10,700 ft3/s), by a factor of 1.6. The 

median discharge for the entire pre-dam period from 

May 8, 1921, through March 12, 1963, was 7,980 ft3/s. 

Although large floods were common in the pre-dam river, 

discharges exceeded 18,500 ft3/s only about 25 percent of 

the time. Topping and others (2000) showed that sand 

accumulated substantially in the pre-dam river in Marble 

and upper Grand Canyons only when the discharge was 

lower than about 9,000 ft3/s, and that sand eroded from 

Marble and upper Grand Canyons when the discharge 

was higher than about 16,000 ft3/s. The pre-dam decadal 

variation in discharge therefore had major implications for 

sand storage in Marble and upper Grand Canyons; the 

1920s were likely the pre-dam decade most dominated by 

erosion of sand from this reach, and the 1930s were likely 

the pre-dam decade most dominated by accumulation of 

sand in this reach. 

During the pre-dam era, the discharge of the river 

was fairly steady over sub-daily time scales, with a 

median daily range in discharge of only 542 ft3/s. 

Therefore, daily mean discharges provide a reasonable 

characterization of the hydrology of the pre-dam river. 

An exception is the summer thunderstorm season, when 

the daily range in discharge was observed to exceed 

30,000 ft3/s, but these extreme daily ranges occurred on 

average only during 1 day in every 3 years. Daily ranges 

in discharge were larger than 10,000 ft3/s only during 

1 percent of all pre-dam days. The median daily range 

in discharge was greatest during the June part of the 

snowmelt flood (3,230 ft3/s). The decadal daily range in 

discharge was somewhat correlated with the decadal 

discharge. As in the case of discharge, the median daily 

range in discharge during the driest decade, the 1930s 

(516 ft3/s), differed from the median discharge during the 

wettest decade, the 1920s (808 ft3/s), by a factor of 1.6. 

The operation of Glen Canyon Dam has removed 

the seasonality from discharge and from the daily range 

in discharge. Dam operations have removed flood flows 

and base flows, and dam operations for hydroelectric 

power generation have introduced wide-ranging daily 

fluctuations in discharge that are relatively consistent 

throughout the year. The changes that dam operations 

have imposed on the hydrology of the Colorado River at 

Lees Ferry exceed anything in the quasi-natural pre-dam 

period of record. 

As for the pre-dam river, discharge and the daily 

range in discharge have varied substantially over decadal 

time scales in the post-dam river as dam operations 

have evolved in response to the filling of Lake Powell 

reservoir and to growing concern about the effects of dam 

operations on the downstream Colorado River ecosystem. 

From the 1960s to the 1990s, the median discharge of the 

Colorado River at Lees Ferry increased from 9,490 ft3/s 

to 13,500 ft3/s. During the wettest post-dam decade, the 

1980s, the median discharge of the Colorado River at 

Lees Ferry was 15,900 ft3/s, only slightly higher than 

that during the 1990s. During the four post-dam 

decades, discharges under which sand can accumulate 

in Marble and upper Grand Canyons have progressively 

disappeared. During the 1960s, discharge was greater than 

9,000 ft3/s 52.7 percent of the time; during the 1970s, 
58 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

discharge was greater than 9,000 ft3/s 62.2 percent of the 

time; during the 1980s, discharge was greater than 

9,000 ft3/s 75.5 percent of the time; and during the 1990s, 

discharge was greater than 9,000 ft3/s 82.6 percent 

of the time. Through time, the post-dam river has been 

progressively dominated by discharges that erode sand, 

and base flows have disappeared. Discharges most likely 

to erode sand from Marble and upper Grand Canyons 

occurred during the 1990s. 

Dam operations for power generation have 

increased the median daily range in discharge by a factor 

of 15.8 relative to the pre-dam median daily range in 

discharge. The post-dam median daily range in discharge, 

8,580 ft3/s, actually exceeded the pre-dam median 

discharge of 7,980 ft3/s. As a result of this increase in the 

sub-daily variability in discharge, daily mean discharges 

no longer provide an adequate characterization of the 

hydrology of the Colorado River. The decade with the 

largest daily range in discharge was the reservoir-filling 

decade of the 1970s, when the median daily range in 

discharge was 13,700 ft3/s (a factor of 25.2 greater than 

the pre-dam median daily range in discharge). The decade 

with the smallest daily range in discharge was the 1990s, 

when the median daily range in discharge was 4,940 ft3/s 

(a factor of 9.1 greater than the pre-dam median daily 

range in discharge). Relative to the pre-dam period of 

record, dam operations for hydroelectric-power 

generation have increased the daily range in discharge 

during all but 0.1 percent of all days. Prior to closure of 

the dam, the daily discharge range exceeded 10,000 ft3/s 

on only about 1 percent of all days. During the post-dam 

period, the daily discharge range exceeded 10,000 ft3/s on 

about 43 percent of all days. 

The operations of Glen Canyon Dam also have 

altered the frequency of floods on the Colorado River at 

Lees Ferry. Prior to closure of the dam, the average 

recurrence intervals were 1 year for floods with peak 

discharges of 50,000 ft3/s, and 6 years for floods with 

peak discharges of 120,000 ft3/s. Although dam 

operations have maintained the pre-dam frequency of 

floods with peak discharges of about 29,000 ft3/s, the 

frequency of floods with peak discharges greater than this 

value has been reduced, whereas the frequency of floods 

with peak discharges less than this value has been 

increased. For example, the 2-year flood during the predam period was 85,000 ft3/s whereas the 2-year flood 

during the post-dam period was 31,500 ft3/s. Because of 

this increase in the frequency of smaller "floods," the 

annual flood series is inadequate to characterize the floodfrequency distribution in the post-dam river. During the 

pre-dam period of record, the average recurrence interval 

for floods with peak discharges of 20,000 ft3/s was 97 

days. After closure of the dam, these 20,000-ft3/s floods 

increased in frequency by a factor of 27, so that their 

recurrence interval is now 3.6 days. In addition to 

increasing the frequency of these lower floods, operation 

of the dam has also resulted in the longest periods of 

sustained high discharge; all four of the longest periods of 

sustained discharge in excess of 18,500 ft3/s occurred 

after closure of the dam.

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APPENDIX A: List of Accession, Box, and Location

Numbers where the U.S. Geological Survey

Data used in this Study are Stored in the

Federal Records Centers

Appendix A 63

Access to these data must be requested through the 

District Chief, Arizona District, USGS,Tucson, Arizona.

Files Stored in the Federal Records Center in 

Laguna Niguel, California

1921–37 Colorado River at Lees Ferry surface water records 

file: Accession # 57-78-0006, Box # 2 of 2, Location 

# MB053635

1921–34 Colorado River at Lees Ferry gage height 

observations: Accession # 57-68A-0138, Box # 6 of 6, 

Location # SB214830

1923–48 Colorado River at Lees Ferry stage-recorder charts: 

Accession # 57-55A-584, Box # 5 of 34, Location 

# RB4957

1949–56 Colorado River at Lees Ferry stage-recorder charts: 

Accession # 57-60-336, Box # 16 of 48, Location 

# SB025762

1966–70 Colorado River at Lees Ferry annual technical files 

and stage-recorder charts: Accession # 57-74A-1590, 

Box # 1 of 59, Location # MB8995

1971–72 Colorado River at Lees Ferry stage-recorder charts: 

Accession # 57-76A-5301, Box # 1 of 39, Location 

# YB34320

1973 Colorado River at Lees Ferry annual technical file and 

stage-recorder chart: Accession # 57-76A-5301, Box # 1 

of 39, Location # YB34320

1974–77 Colorado River at Lees Ferry annual technical files 

and stage-recorder charts: Accession # 57-82-0004, 

Box # 1 of 4, Location # MB047919

1978–79 Colorado River at Lees Ferry annual technical files 

and stage-recorder charts: Accession # 57-94-0030, 

Box # 13 of 13, Location # SB13994

1980 Colorado River at Lees Ferry annual technical file and 

stage-recorder chart: Accession # 57-94-0030, Box # 1 of 

13, Location # SB13994

1981 Colorado River at Lees Ferry annual technical file and 

stage-recorder chart: Accession # 57-94-0030, Box # 13 of 

13, Location # SB13994

1982–89 Colorado River at Lees Ferry annual technical files 

and stage-recorder charts: Accession # 57-94-0030, 

Box # 1 of 13, Location # SB13994

1990 Colorado River at Lees Ferry annual technical file and 

stage-recorder chart: Accession # 57-96-0010, Box # 3 of 

4, Location # BT2670

1921–28 Colorado River at Lees Ferry discharge measurement 

notes: Accession # 57-55-0584, Box # 10 of 58, Location 

# BT000964

1929–31 Colorado River at Lees Ferry discharge measurement 

notes: Accession # 57-55-0584, Box # 11 of 58, Location 

# BT000964

1932–34 Colorado River at Lees Ferry discharge measurement 

notes: Accession # 57-55-0584, Box # 12 of 58, Location 

# BT000964

1935–37 Colorado River at Lees Ferry discharge measurement 

notes: Accession # 57-55-0584, Box # 13 of 58, Location 

# BT000964

1938–41 Colorado River at Lees Ferry discharge measurement 

notes: Accession # 57-55-0584, Box # 14 of 58, Location 

# BT000964

1942–48 Colorado River at Lees Ferry discharge measurement 

notes: Accession # 57-55-0584, Box # 15 of 58, Location 

# BT000964

1949–57 Colorado River at Lees Ferry discharge measurement 

notes: Accession # 57-60-0336, Box # 3 of 48, Location 

# ST007405

1921–28 Colorado River at Lees Ferry discharge measurement 

notes: Accession # 57-55-0584, Box # 10 of 58, Location 

# BT000964

1930–34 Colorado River at Lees Ferry sediment records: 

Accession # 57-78-0005, Box # 1 of 26, Location 

# MT016349

1948–53 Colorado River at Lees Ferry sediment records: 

Accession # 57-78-0005, Box # 1 of 26, Location 

# MT016349

1954–59 Colorado River at Lees Ferry sediment records: 

Accession # 57-78-0005, Box # 2 of 26, Location 

# MT016349

1960–76 Colorado River at Lees Ferry sediment records: 

Accession # 57-78-0005, Box # 3 of 26, Location 

# MT016349

1891–37 Colorado River at Yuma surface water records file: 

Accession # 57-78-0006, Box # 2 of 2, Location 

# MB053635
64 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

Files Stored in the Federal Records Center in 

Denver, Colorado

1921–26 Colorado River at Lees Ferry discharge revision, 

original computations: Accession # 57-64A-0423, 

Box # 3 of 24, Location # 62480

1921–26 Colorado River at Lees Ferry annual technical files: 

Accession # 57-64A-0423, Box # 2 of 24, Location 

# 62479

1927–50 Colorado River at Lees Ferry annual technical files: 

Accession # 57-64A-0423, Box # 3 of 24, Location 

# 62480

1951–60 Colorado River at Lees Ferry annual technical files: 

Accession # 57-66A-0574, Box # 21 of 27, Location 

# 225838

1961–65 Colorado River at Lees Ferry annual technical files: 

Accession # 57-70B-117, Box # 13, Location # 904128

1957–60 Colorado River at Lees Ferry stage-recorder charts: 

Accession # 57-67A-0261, Box # 3 of 13, Location 

# 327910

1961-–65 Colorado River at Lees Ferry stage-recorder charts: 

Accession # 57-70A-117, Box # 1, Location # 904104

Colorado River near Grand Canyon gaging station 

reconnaissance and construction reports: Accession 

# 57-64A-0423, Box # 1 of 24, Location # 62478

1941 Paria River at Lees Ferry annual technical file: Accession 

# 57-66A-0574, Box # 2 of 27, Location # 225819
APPENDIX B: Important Events in the

History of the Lees Ferry Gaging Station

Appendix B 67

May 8, 1921 E.C. LaRue installed a temporary inclined staff gage on the right bank (fig. 1C). This gage is 

herein referred to as the "LaRue Gage." This gage was not graduated and was read twice 

daily by measuring down with a tape from a nail used as a reference point. H.A. Schenck 

(Chief of Surveys, Southern California Edison Company) later told Roger C. Rice (District 

Engineer, USGS) that the slope of this gage was approximately 2:1 (which corresponds to an 

angle of about 26°34′). E.C. LaRue later said that he thought the slope of the gage was closer 

to 32°. The first reading of stage was made at 6:00 p.m.

June 12–13, 1921 The LaRue Gage was overtopped by water between 5 p.m. on June 12 and 6:00 a.m. on 

June 13. Frank T. Johnson, the observer, continued to measure the stage twice daily after the 

gage was overtopped by "pegging" up the slope above the gage. This method consisted of 

inserting a stick at the edge of water and then later measuring the change in stage on the stick 

and inserting a new stick at the new edge of water. G.S. Stevens (in a memorandum dated 

March 15, 1924, in Appendix D) later estimated that the first two "pegged" measurements, 

because of their much larger magnitude, were probably measurements up the sloping bank 

and were not vertical measurements.

June 13, 1921 Roger C. Rice (USGS) drafted a letter to H.W. Dennis (Construction Engineer, Southern 

California Edison Company) detailing the plan of cooperation between the USGS and the 

Southern California Edison Company for the establishment of the Lees Ferry gaging station. 

Under this agreement, the USGS would serve in a general advisory capacity, provide all 

current-meter equipment, and would process all of the data collected at the site. The 

Southern California Edison Company would then assume responsibility for the cost of all 

gage-related construction, and would pay two resident hydrographers to make gage-height 

observations, discharge measurements, and perform maintenance.

June 18, 1921 Peak of the June 1921 flood occurred at 2:00 p.m. Frank T. Johnson marked the high-water 

elevation as the top of nail in vertical board on the south side of the road near a wash. As the 

flow receded, Johnson continued to record the stage twice daily by "pegging" down the 

slope.

June 21, 1921 H.A. Schenck (Southern California Edison Company) and Roger C. Rice (USGS) met in 

Flagstaff, Arizona, to discuss the details of establishing the Lees Ferry gaging station.

June 22, 1921 Arrangements were made with the Flagstaff Lumber Company to furnish the material for the 

A-frames and cable car for the discharge-measurement cableway. Arrangements were made 

with P.S. Solberg, 309 South Beaver Street, Flagstaff, to do the carpentry.

June 23, 1921 H.A. Schenck and Roger C. Rice visit Lees Ferry with survey party. Elevation of high-water 

mark near the future site of the permanent Lees Ferry Gage at the dugway was determined. 

They approved the location for the discharge measurement cableway that was previously 

selected by E.C. LaRue. This location is marked as the "Upper Cableway" in fig. 1C, and is 

herein referred to as the Upper Cableway.

June 23, 1921 Jerry Johnson told Roger Rice that the peak stage of the 1884 flood was 2 ft higher than the 

peak stage of the 1921 flood.

June 24, 1921 The flow receded to below the top of the LaRue Gage. Frank T. Johnson found that the 

lower part of the gage had broken loose and that the top part had rotated out of position. He 

installed a new temporary inclined staff gage, with a new slope of 24°35′. This gage became 

known as the "Number 1 Gage." Johnson stated that he thought the reference-point nail for 

the Number 1 Gage was at the same elevation as the reference point for the LaRue 

Gage, but he was not completely sure of his observation. G.C. Stevens later estimated in a 

memorandum dated November 22, 1923 (Appendix D), that the reference point for the new 
68 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

Number 1 Gage was probably 2.65 ft lower than that for the LaRue Gage. Based on 

information presented in Appendix D, the reference point for the Number 1 Gage was most 

likely about 3.65 ft lower than that for the LaRue Gage.

June 24, 1921 Due to the high velocities encountered at Lees Ferry during the snowmelt flood, the decision 

was made to use heavier lead weights than normal when making discharge measurements. 

One 100-pound weight and one 50-pound torpedo-shaped weight were thus ordered from 

W. and L.E. Gurley, Troy, New York. At that time, the USGS only had weights lighter than 

30 pounds in stock. Also, a special winch was designed to handle the heavier weights. 

Because of the heavier equipment, the cable and cable car were also designed to be larger.

June 25, 1921 First reading on the new Number 1 Gage made by Frank T. Johnson at 7 a.m.

June 25, 1921 F.S. Solberg of the Flagstaff Lumber Company informed Schenck and Rice that the wooden 

A-frame for the Upper Cableway would be completed by July 8, 1921.

June 28, 1921 Temporary inclined staff gage installed on the right bank just above the ferry crossing (fig. 

1C). This gage was known as the "Number 2 Gage" and had a slope of 24°20′. The first stage 

measurement on this gage was made at 4:30 a.m. The Number 2 Gage was read twice daily 

until 6:00 a.m. on August 11, when it was ordered removed by E.C. LaRue. The datum of 

this gage was never tied into the datums of the other gages.

July 7, 1921 Temporary inclined staff gage installed on the left bank near the large rock below the Upper 

Cableway site. This gage was known as the "Number 3 Gage" and was at the site of the 

higher-water inclined sections of the future "Number 4 Gage" (fig. 1C). The first stage 

measurement on this gage was made at 7:45 a.m. The Number 3 Gage was read twice daily 

until 9:05 a.m. on August 11, when it was ordered removed by E.C. LaRue. The datum of 

this gage was never tied into the datums of the other gages.

July 25, 1921 Southern California Edison Company began construction of the Upper Cableway for 

measuring discharge.

August 3, 1921 Temporary inclined staff gage installed on the left bank near the Number 3 Gage. This gage 

was first read at 6:10 a.m. and became known as the "Number 4 Gage" (fig. 1C).

August 3,1921 First discharge measurement made from the Upper Cableway.

August 5, 1921 Temporary inclined staff gage installed at the end of the dugway (fig. 1C). This gage became 

known as the "Dugway Gage" and was located immediately upstream from the future 

permanent Lees Ferry Gage. The datum of the Dugway Gage was the same as the future 

Lees Ferry Gage.

August 9, 1921 W.E. Johnson replaced Frank T. Johnson as the gage observer.

August 11, 1921 Number 2 and Number 3 Gages ordered removed by E.C. LaRue.

August 14, 1921 First dependable stage measurement made at the Dugway Gage at 8:50 a.m. 

September 19, 1921 Eyebolt failed on Upper Cableway and cable car fell in river at 3:00 p.m. with I.G. Cockcroft 

and Elmer Johnson aboard. Cockcroft and Johnson swam to shore. Johnson was not a strong 

swimmer and used the car top as a life preserver.

September 21, 1921 Cable reattached and cable car placed back on cable (fig. B1). Discharge measurements 

were not made from the cable again until December 3, 1921. Discharge from October 4 

through November 30 was measured from a boat at the Upper Cableway cross-section.
Appendix B 69

A.

B.

Figure B1. Repairs to the Upper Cableway and the highwater marks from the 1921 flood (photographs by R.C. Rice, 

U.S. Geological Survey, September 20, 1921). (A) View from 

the right bank of the cable and cable car in the river in the 

morning. High-water marks from the June 1921 flood are 

visible on the left bank. Source of this and the photographs 

in figs. B1B–B3D, and B4–B6: 1921–37 Surface Water 

Records File, Colorado River at Lees Ferry, Arizona, file 

stored at the Federal Records Center in Laguna Niguel, 

California, in Accession No. 57-78-0006, Box 2 of 2, 

Location No. MB053635. (B) Downstream view from the 

right bank of Sid Wilson attaching the chain come-along to 

the cable. High-water marks from the June 1921 flood are 

visible on the left bank in the background. 
70 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

C.

D.

Figure B1—Continued. Repairs to the Upper 

Cableway and the high-water marks from the 1921 flood 

(photographs by R.C. Rice, U.S. Geological Survey, 

September 20, 1921). (C) View from the right to the left 

bank of the cable car rescued from the river. High-water 

marks from the June 1921 flood are visible below the 

level of the road on the left bank. Numbered are: 1. H.W. 

Dennis (Southern California Edison Company) and 2. R.Q. 

Grant (U.S. Weather Bureau). (D) View from the right 

bank of Sid Wilson reattaching the cable car to the Upper 

Cableway. High-water marks from the June 1921 flood 

are visible below the level of the road on the left bank. 
Appendix B 71

September 21, 1921 Dugway Gage was permanently installed and consisted of a vertical low-water section from 

0 ft to 9.5 ft, inclined middle section from 8 ft to 21 ft, and a high-water section from 21 ft to 

28 ft. Elevation of the peak of the June 1921 flood was found to be 26.5 ft on this new gage 

(fig. B2). First stage measurement made at 6:25 p.m. The datum of the Dugway Gage was 

3,106.16 ft above the NGVD1929.

September 24–29, 1921 New Number 4 Gage was permanently installed. This gage replaced the old temporary 

Number 4 Gage on the large rock in the channel and the inclined Number 3 Gage on the left 

bank. This gage consisted of a vertical low-water section attached to the large rock in the 

channel near the left bank below the Upper Cableway, and five higher-elevation inclined 

sections on the left bank. Vertical section read from 3 ft to 15 ft, and inclined sections read 

from 12.5 ft to 33.5 ft (fig. B3). Elevation of the road on the left bank was at a stage of 27 ft. 

Photographs indicate that, at the Number 4 Gage, this road was just above the elevation of 

the peak stage of the June 1921 flood. The datum of the Number 4 Gage was 3,106.82 ft 

above the NGVD1929.

September 25–26, 1921 Number 1 Gage was permanently installed and consisted of a vertical low-water section 

from 10.15 ft to 13.55 ft and an inclined higher-water section that extended from 13.5 ft to 

23 ft. Low-water section was located about 117 ft downstream from the inclined section 

(fig. B4). Elevation of the peak of the June 1921 flood was found to be 30.89 ft on the 

Number 1 Gage. The datum of the Number 1 Gage was 3,102.60 ft above the NGVD1929.

September 29, 1921 Old temporary Number 4 Gage on rock was removed.

September 30, 1921 The USGS computed daily mean discharge through this date from the stages measured at the 

Number 1 Gage. Because of uncertainties in how the stages measured on the LaRue Gage 

related to those measured on the Number 1 Gage, no daily mean discharges were published 

by the USGS prior to June 13, 1921.

October 1, 1921 The USGS computed daily mean discharge after this date from the stages measured at the 

Dugway Gage.

June 7, 1922 I.G. Cockroft and Elmer Johnson measured the elevation of two high-water marks from the 

June 1921 flood at the Lonely Dell Ranch. The first of these marks was "about 8 inches 

above the ground on post of fence near bottom wire" and was shown to Cockcroft and Elmer 

Johnson by Jerry Johnson. The corrected NGVD1929 elevation of this high-water mark was 

3,132.64 ft, which equals 26.48 ft above the datum of the Dugway Gage. This high-water 

mark was only 0.02 ft lower than that measured on the left bank at the Dugway Gage. The 

second of these high-water marks was at the high-water edge of the alfalfa field near the 

fence and road. This high-water mark was traced to be within 50 ft of the original gage on 

the Paria River (this gage was one meander upstream, about 1,970 ft, from the current Paria 

River gage). The high-water mark consisted of small pieces of drift and grass. The corrected 

NGVD1929 elevation of this high-water mark was 3,132.61 ft, which equals 26.45 ft above 

the datum of the Dugway Gage. This mark was only 0.05 ft lower than that measured at the 

Dugway Gage. These high-water marks were located between 0.6 and 1.2 miles diagonally 

downstream on the right bank of the Colorado River, and up the Paria River valley. In a 

streamwise coordinate system, these marks would be located about 0.3 miles downstream 

from the Dugway Gage. Therefore, the slope of the water surface in the reach immediately 

below the Dugway Gage was approximately 0.00001 to 0.00003 during the peak of the 1921 

flood.
72 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

A.

B.

Figure B2. The Dugway Gage and the high-water mark from 

the 1921 flood (photographs by R.C. Rice, U.S. Geological 

Survey, September 22, 1921). (A) Downstream view of the 

vertical low-water section (on right) and inclined middle 

section (on left) of the Dugway Gage. (B) Downstream view 

of the inclined middle section of Dugway Gage. 
Appendix B 73

June 7, 1922 I.G. Cockcroft and Elmer Johnson first tied the elevation of the July 7, 1884, flood peak into 

the Dugway Gage. The 1884 flood was the most recent flood prior to the June 1921 flood 

that was larger than the 1921 flood. The high-water mark was a crotch in a peach tree at the 

Lonely Dell Ranch (on river right below the Dugway Gage) from which Jerry Johnson 

rescued a cottontail rabbit. The corrected NGVD1929 elevation of this mark was 3,136.90 ft, 

which equals 30.74 ft above the datum of the Dugway Gage. The elevation of the stump of 

this tree near Johnson’s house was checked by levels by W.E. Dickinson on September 22, 

1924. On April 13, 1938, J.S. Gatewood estimated that the peak stage of the 1884 flood 

would have been at a stage of about 31.5 ft on the Dugway Gage. However, because the 

water-surface slope during the 1921 flood was between 0.00001 and 0.00003 between the 

Dugway Gage and the Lonely Dell Ranch, and it is likely that the water-surface slope was 

even lower during the 1884 flood (because of flattening of the water surface in this reach 

with increasing stage, as shown in this study), the peak stage of the 1884 flood was probably 

no more than 31 ft on the Dugway Gage.

August 10, 1922 I.G. Cockcroft installed a new vertical low-water section for the Dugway Gage to replace the 

old one that had "torn loose." Cockcroft stated that the "new vertical reads 1/2 tenth lower." 

Roger C. Rice stated that Cockcroft probably installed this new staff gage slightly lower than 

the original one so that the readings on it and on the lower end of the inclined middle section 

would agree. Although the original vertical low-water and inclined upper sections were 

installed relative to the same datum, stage readings tended to be about 0.05 ft lower on the 

original vertical low-water section than on the inclined middle section.

C.

Figure B2—Continued. The Dugway Gage and the high-water mark from the 1921 flood (photographs by R.C. Rice, U.S. Geological Survey, 

September 22, 1921). (C) Downstream view of the high-water section of Dugway Gage painted on cliff next to dugway road. I.G. Cockcroft standing next 

to gage. High-water mark from the June 1921 flood visible on the cliff at a stage of 26.48 feet on the gage (determined by levels on June 7, 1922). This 

stage equals an elevation of 3,132.64 feet above the NGVD1929. 
74 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

A.

B.

Figure B3. The Number 4 Gage and the high-water marks from the 1921 flood. (A) October 2, 1921, downstream view of the of the Upper Cableway leftbank A-frame and the Number 4 Gage. Position of inclined sections of Number 4 Gage on the left bank indicated; vertical low-water section of the Number 

4 Gage is on the large rock in the river near the left bank. High-water marks from the June 1921 flood are visible on both banks. (This photograph and those 

in parts B through D taken by R.C. Rice of the U.S. Geological Survey.) (B) October 2, 1921, downstream view of the Number 4 Gage and high-water marks 

from the June 1921 flood. Inclined upper sections of the gage are on the left bank; vertical low-water section is on the rock to the right. Two people and a 

boat are visible in the foreground for scale. 
Appendix B 75

September 20, 1922 Last regular stage measurement made at the Number 4 Gage at 7:20 a.m.

October 10–

December 16, 1922 The concrete gagehouse and stilling well for the permanent Lees Ferry Gage were built 

immediately downstream from the Dugway Gage. The new Lees Ferry Gage and the 

Dugway Gage both have the same datum.

January 19, 1923 An Au-fuzee continuous water-stage recorder was installed.

September 1, 1923 USGS takes over full control of the Lees Ferry gaging station from the Southern California 

Edison Company.

September 22, 1923 Last stage measurement made at the Number 4 Gage.

April 1924 Inclined slope gage installed on the left bank at the Upper Cableway. This gage was known 

as the "Cable Gage." Gage read from 13.5 ft to 34 ft (fig. B5). Gage was used to help 

constrain the change in stage during discharge measurements at the Upper Cableway (which 

was 1 mile upstream from the recorder gage). Gage was tied into a benchmark near the 

Number 4 Gage on April 24, 1924. The original datum of the Cable Gage was 3,109.40 ft 

above the NGVD1929.

April 17, 1924 Inclined upper section of the Number 1 Gage replaced.

June 1, 1924 New inclined upper section of the Number 1 Gage tied into the old Southern California 

Edison Company benchmark.

August 26–27, 1924 New vertical low-water section installed about 20 ft offshore from the new inclined upper 

section of the Number 1 Gage. This new vertical section was anchored to the offshore side of 

a large rock, and was tied in with a rod and level to the datum of the Number 1 Gage.

C.

Figure B3—Continued. The Number 4 Gage and the high-waters mark from the 1921 flood. (C) October 2, 1921, view of the vertical low-water 

section of the Number 4 Gage on the rock. High-water marks from June 1921 flood are visible on the right bank. 
76 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

October 12, 1924 Cable Gage reconstructed and remarked. Datum of gage was lowered 10 ft. New datum of 

the Cable Gage was 3,099.40 ft above the NGVD1929.

October 5, 1925 Backwatered flow conditions develop in the Colorado River at the Lees Ferry Gage as a 

result of a large flood on the Paria River. The stage of the Colorado River at the Lees Ferry 

Gage increased by as much as 1 ft as a result of the Paria River flood entering the Colorado 

River just downstream from the gage. Backwatered flow conditions lasted 2 hours before an 

increase in the discharge of the Colorado River increased the stage above 11 ft at the Lees 

Ferry Gage. 

October 12, 1925 Floating logs tore out vertical low-water section of the Number 1 Gage. Gage useless below 

13.5 ft.

October 19, 1925 Last stage measurement made at the Number 1 Gage at 6:00 a.m.

D.

E.

Figure B3—Continued. The Number 4 Gage and the high-waters mark from the 1921 flood. (D) October 2, 1921, view of the inclined upper 

sections of the Number 4 Gage. Analysis of figs. B1D, 3A, and 3B indicates that the peak stage of the June 1921 flood was at about the elevation 

of the road in this photograph. This elevation equals a stage of 27 feet on the Number 4 Gage, which equals an elevation of 3,133.9 feet above 

the NGVD1929. (E) May 25, 1999, view of the remnants of the inclined portion of the Number 4 Gage. Photograph taken by D.J. Topping ( U.S. 

Geological Survey)
Appendix B 77

A.

B.

Figure B4. The Number 1 Gage (photographs by R.C. Rice, U.S. Geological Survey, September–October 1921). (A) Downstream view of the inclined higherwater section of the Number 1 Gage on about September 20, 1921. This was the gage built by the observer, Frank T. Johnson, after the crest of the June 

1921 flood. It was read by taping down from a nail used as a reference point. The trench was dug to connect the gage to the river. The vertical low-water 

enamel section had not yet been installed when this picture was taken. Peak stage of the June 1921 flood was measured at 30.89 feet on this gage. This 

stage equals an elevation of 3,133.50 feet above the NGVD1929. (B) October 3, 1921, downstream view of the vertical low-water section of Number 1 Gage 

installed by Roger C. Rice on September 25, 1921. Elmer Johnson, assistant gage observer and hydrographer, standing next to gage. Location of Dugway 

Gage on the left bank is indicated. 
78 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

January 1929 Lower Cableway completed at wider section between the Upper Cableway and the Lees 

Ferry Gage for making more accurate discharge measurements during periods of higher 

water (fig. 1C). The section chosen was about twice as wide as the section at the Upper 

Cableway. Work on this project commenced with the pouring of the concrete anchorages in 

December 1927 and construction of the A-frames in January 1928. Cable was finally erected 

in January 1929 when the river was low enough to allow easier stringing of the long cable 

across the channel. Originally, it was hoped that the river would completely freeze over, 

making the cable stringing easier. In January 1929, the job was completed when the river at 

the chosen cross-section was restricted to only a narrow channel along the right bank, with 

the remainder of the channel occupied by a wide sand bar.

March 1929 Upper Cableway was replaced and left A-frame of the Upper Cableway was raised 3 ft and 

set on two concrete piers.

March 1929 The dugway road area between the gage house and the cliff was filled with rocks to prevent 

drift from lodging in the opening and endangering the stilling well and gage house during 

periods of high water. This was possible because the road to the old ferry crossing was no 

longer used after the construction of the Navajo Bridge across the Colorado River, 4.3 miles 

downstream.

Figure B5. The right- to left-bank view of the Upper Cableway showing the Cable Gage on the left bank (appears as a vertical white stripe below the leftbank A-frame). High-water marks from the June 1921 flood are still visible 3 years after the flood (compare with same high-water marks photographed in 

September 1921 in fig. B1A). High-water marks from the June 1921 flood appear to be just above the elevation of the top of the Cable Gage. This 

elevation equals a stage of approximately 34.5 feet on the Cable Gage, which equals an elevation of 3,133.9 feet above the NGVD1929. Also shown in 

the foreground is a Price current meter and 75-lb weight suspended from the cable car. Tail of weight bent slightly upward after sounding rocky bottom. 

(Photo taken by W.E.Dickinson, U.S. Geological Survey, May 1924).
Appendix B 79

March–September 1929 Too much water was computed passing the Lees Ferry gaging station relative to the Grand 

Canyon gaging station (minus the inflow from the Paria River, Moenkopi Wash, and the 

Little Colorado River) during this period (unpublished USGS memorandum dated 

December 30, 1929, by W.E. Dickinson; Appendix C). This problem was worse during 

higher flows. On average 2.7 percent too much water was computed passing Lees Ferry 

during water year 1929. Although no correction was made to the water-year 1929 record, 

discharges at the Lees Ferry gaging station during subsequent years would be reduced to 

equal those at the Grand Canyon gaging station when this type of discrepancy occurred. 

Although no official reason for this discrepancy was determined, this discrepancy started 

after the rock fill was placed between the gage and the cliff, and after the Upper Cableway 

was replaced. This discrepancy also also started with the use of different current meters.

August 2, 1929 Backwatered flow conditions develop in the Colorado River at the Lees Ferry Gage as a 

result of a large flood on the Paria River. The stage of the Colorado River at the Lees Ferry 

Gage increased by slightly more than 1 ft as a result of the Paria River flood entering the 

Colorado River just downstream from the gage. This event was the largest backwatering 

event ever recorded at the Lees Ferry Gage due to a Paria River flood. Backwatered flow 

conditions lasted 8 hours before an increase in the discharge of the Colorado River raised the 

stage to greater than 13.9 ft at the Lees Ferry Gage. 

October 1, 1929 The first suspended-sediment samples were collected at Lees Ferry. Samples were collected 

at three locations across the cross-section; no mention was made of which cableway was 

used, but it was probably the Lower Cableway. Sampler used was the old Colorado River 

Sampler described in Howard (1930, 1947).

February 1930 Rock fill extended up to the level of the gage house platform (fig. B6).

May–June 1930 Suspended-sediment laboratory notes for these 2 months state that the samples were 

weighed in Washington, D.C. All subsequent samples were probably analyzed on site at the 

sediment laboratory at Lees Ferry (fig. 1C).

May 24, 1932 Memorandum by J.S. Gatewood describing the suspended-sediment program states that 

samples are taken at three positions across the Lower Cableway. The preferred stations are at 

700, 600, and 450 ft from the left-bank endpoint of the cable, except during extremely low 

water, when station 450 ft is dry. When this occurs, the stations are shifted to the right. 

Samples are not taken every day, but only when someone has to cross the Lower Cableway 

for some other reason. Hydrographer at Lees Ferry, using a balance, weighs all of the empty 

and full bottles, and empty and full filter papers.

June 11, 1932 Lower inclined section of the Cable Gage washed out during a period of high water. It was 

replaced on June 11 with a new temporary section consisting by a 2"x6"x12’ timber on 

which feet and tenths were marked by saw marks and paint. The section of the gage replaced 

extended from 18.5 ft to 25.5 ft. Section below this was also free, but was still below water.

September 11, 1932 Lower inclined section of the Cable Gage permanently repaired during the early part of 

September, with levels run on September 11. The new section extends from 16 ft to 25 ft on 

the slope. From 13.5 ft to 16 ft, the gage is attached to a vertical post driven as far as possible 

down into the bank.

May–June 1933 Both wooden A-frames on the Lower Cableway were replaced with steel A-frames. Steel 

landing towers were built at the left end of the Upper Cableway and at both ends of the 

Lower Cableway.
80 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

Figure B6. March 1933 downstream view of the Lees Ferry Gage and the high level of 

the rock fill placed behind the gage in 1929–30. The inclined middle section of the old 

Dugway Gage is in the foreground; compare to view in fig. B2B. (Photo taken by the U.S. 

Geological Survey.)
Appendix B 81

December 16, 1933 Suspended-sediment sampling program discontinued at Lees Ferry.

March 1941 Staff gage installed on the right bank of the Colorado River, 1.5 miles downstream from the 

Lees Ferry Gage (fig. B7). This gage was downstream from the gravel bar and downstream 

from the riffle below the Paria River. The USGS referred to this gage as both the "Lower 

Paria Gage" and the "staff gage on the Colorado River below the mouth of the Paria River." 

This gage is herein referred to as the "Lower Staff Gage." It consisted of five enamel sections 

reading from 0 ft to 16 ft fastened to 2"x6" timber bolted to the vertical rock cliff. The 

original survey of this gage (a 4-mile circuit of levels) conducted in 1941 determined that the 

datum of the gage was 3,094.05 ft above the NGVD1929. Resurvey of the gage benchmark 

by Grand Canyon Monitoring and Research Center staff in the 1990s indicated that the 

actual datum of this gage was 3,093.98 ft above the NGVD1929.

November 10, 1942 Suspended-sediment sampling program restarted at Lees Ferry. Samples on this day were 

collected by both throwing the sampler out from the bank and by deploying the sampler 

from a boat.

November 12, 1942 First suspended-sediment sample collected from the cable. After this date, each suspendedsediment measurement consisted of only one vertical in the cross-section. Measurements 

were collected typically every 2 days through the end of September 1944.

September 29, 1944 Suspended-sediment sampling program discontinued at Lees Ferry. This date marks the end 

of the use of the old Colorado River Sampler at Lees Ferry.

December 19, 1944 A large rockfall covered the right-bank anchor of the Upper Cableway. This rockslide caused 

about 20–25 ft of permanent channel narrowing at the cableway cross-section (fig. 7C in 

Topping and others, 2000). Rock was removed from the cable during February and March 

1945. Cable was stretched somewhat, but otherwise appeared undamaged.

March 29, 1945 Excess sag due to the rockfall removed from the Upper Cableway.

October 1, 1947 Daily suspended-sediment sampling program started at Lees Ferry. All samples were 

collected using the new D-43 depth-integrating sampler. Samples were collected at four 

stations across the cross-section at the Lower Cableway. The preferred stations were at 330, 

510, 620, and 730 ft from the left-bank endpoint of the cable. Frequently, the station at 330 ft 

was dry. All samples were processed for concentration in the field laboratory at Lees Ferry 

using standard evaporation procedures. Grain-size analyses were performed on singlevertical samples episodically collected in the center of the channel starting in July 1949. 

These samples were processed for grain size at the laboratory in Albuquerque, New Mexico.

November 1947 New sediment laboratory at Lees Ferry completed.

 February 14, 1951 Suspended-sediment sampling program moved to the Upper Cableway. All samples 

continued to be collected using a D-43 sampler. Each measurement consisted of three 

samples collected at centroids of equal discharge across the cross-section.

April 1, 1951 The methodology of collecting the subset of suspended-sediment samples processed for 

grain-size analysis was changed. Grain-size analyses were now performed on a composite of 

the three samples collected at centroids of equal discharge across the cross-section.
82 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

Figure B7. November 13, 2000, downstream view of the remnants of the Lower Staff Gage. Note 

the ladder rungs attached to the cliff; the Lower Staff Gage was located immediately to the right of 

this ladder. Stage of the gage benchmark was 17.5 feet on the Lower Staff Gage. (N.J. Hornewer, 

U.S. Geological Survey, shown for scale, photograph taken by D.J. Topping, U.S. Geological Survey.)
Appendix B 83

September 21, 1951 Replaced Au stage recorder with a new Stevens A35BT stage recorder.

June 1954 Replaced Stevens A35BT stage recorder with a Stevens A35B stage recorder.

July 1, 1957 Memorandum from Dean Tidball (USGS resident Hydrographer at Lees Ferry) to J.M. Stow 

(USGS District Chemist, Albuquerque, N.M.) stating that the D-43 sampler was inadequate 

to sample the entire flow depth at Lees Ferry during higher flows. Tidball wrote, "The D-43 

sampler available here proved inadequate for the proper sampling of the Colorado River 

during its higher stages this year and we were obliged to take some samples to only one half 

the actual depth at two or three cable stations part of the time. At stages [discharges] under 

100,000 ft3/s we managed to go to full depth at two of the stations. One of these, [the] station 

[at] 540 [ft from the left-bank endpoint of the cable], required a very fast travel rate for the 

sampler and due to the difficulty in locating the bottom, resulted in the sampler dragging 

enough [on the bottom] to collect an excessive amount of silt [all sand, silt, and clay was 

referred to as silt by the chemists and some hydrographers during this era] on several 

occasions."

January 8–9, 1958 Hydraulic cable car powered by a gasoline engine installed at the Upper Cableway to allow 

use of a 300-lb sounding weight during discharge measurements.

Spring 1958 A P-46 point-integrating suspended-sediment sampler was used for the first time at Lees 

Ferry for routine sampling during higher flows. This sampler was an improvement over the 

D-43 because it was designed for sampling in deeper, higher-velocity flows, and the nozzle 

could be opened and closed.

February 11, 1959 Cofferdam completed at Glen Canyon damsite 15.5 miles upstream from the Lees Ferry 

Gage.

March 13, 1963 Closure of Glen Canyon Dam reduced discharge in the Colorado River at Lees Ferry to 

1,000 ft3/s.

June 12, 1963 Because of excessive sediment in suspension at the Upper Cableway, suspended-sediment 

samples were collected from a boat near the Lower Cableway from June 12, 1963, through 

July 10, 1963. Samples collected at 1,000 ft3/s after July 10, 1963, were collected by wading 

near the Lees Ferry Gage.

1964 Buildup of aquatic growth on the hydraulic control during spring and summer due to the 

clear water released from Glen Canyon Dam first observed to cause an increase in roughness 

in the discharge measurements. Most suspended-sediment samples were once again 

collected from the Upper Cableway.

October 1965 A D-49 suspended-sediment sampler was now used instead of a D-43 sampler.

February 1965 Lower Cableway disassembled and reinstalled 50 ft upstream from the Lees Ferry Gage. 

This new cableway is herein referred to as the "Modern Cableway."

April–June 1965 The P-46 suspended-sediment sampler was used during the higher flows released from Glen 

Canyon Dam during these months.

August 13, 1965 Daily suspended-sediment sampling program discontinued at Lees Ferry.

October 1965 Suspended-sediment samples now collected on a weekly basis at Lees Ferry.

October 30, 1965 First discharge measurement made at Modern Cableway 50 ft upstream from the Lees Ferry 

Gage.

December 1, 1966 Last discharge measurement made at the Upper Cableway.

February 2, 1967 Digital stage recorder installed in the gage house. This recorder replaces the old Stevens 

analog chart recorder as the primary stage record.
84 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

October 1967 Weekly suspended-sediment sampling program moved from the Upper Cableway to the 

Modern Cableway. Upper Cableway disassembled sometime after this date.

August 4, 1976 Residency of the USGS hydrographer at Lees Ferry discontinued.

1977 Gravel pit operating on the gravel bar at the mouth of Paria River.

January 4–

February 3, 1977 New boat ramp built at Lees Ferry. This reduced the size of an eddy that existed in the right 

portion of the channel under the cableway.

March 10, 1977 Installation of Convertible Data Collection Platform (CDCP) in gage house. The antenna 

and two solar panels used to recharge the CDCP batteries were mounted on a mast on the 

gage house roof. The site address was 16C225534 and the transmission interval was 3 hours.

October 1977 Suspended-sediment sampling frequency changed from weekly to monthly.

April 17, 1982 Cableway struck by helicopter. Pilot and two passengers killed, another hospitalized. No 

serious damage to cable or support structure.

February 8, 1984 USGS removed gasoline engine and other hydraulic gear from cable car. 

November 12, 1986 LaBarge DCP was replaced with a Handar 524 DCP.

March 26, 1987 Wildfire burned 21 acres of tamarisk in the vicinity of the right-bank A-frame of the Modern 

Cableway. Cable car burned beyond repair, landing platform destroyed by fire, A-frame 

paint damaged, cable undamaged.
APPENDIX C: December 30, 1929,

Memorandum of W.E. Dickinson

Appendix C 87
88 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000
APPENDIX D: Relationship between the

Datums of the LaRue and Number 1 Gages

Appendix D 91

Stages measured on the LaRue Gage prior to it being destroyed during the June 1921 flood were related to the 

datum of the Number 1 Gage through a three-step process. First, the January 25, 1922, analysis of Roger C. Rice (below) 

was used for the receding-limb portion of the 1921 flood record, when Frank Johnson "pegged" down the slope. Second, 

the November 22, 1923, analysis of G.C. Stevens (below) was used for the rising-limb portion of the 1921 flood

record, when Frank Johnson "pegged" up the slope after the LaRue Gage was overtopped by water. Third, comparison of 

the instantaneous discharge record at Lees Ferry with the discharge records from upstream and downstream gaging 

stations (fig. 18) suggests that Frank Johnson accumulated an error in stage of approximately 1 ft while he "pegged" up 

the slope during the rising limb of the 1921 flood. In this study, we have distributed this error equally among Johnson’s 

measurements. When combined with the November 22, 1923, analysis of G.C. Stevens, this distributed 1-ft error 

suggests that the reference point for the LaRue Gage was 3.65 ft higher than the reference point for the Number 1 Gage.
92 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

January 25, 1922, memorandum of Roger C. Rice
Appendix D 93
94 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000
Appendix D 95

November 22, 1923, memorandum of G.C. Stevens
96 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000
APPENDIX E. Days with Disagreement Greater than or

Equal to 5 Percent between the Daily Mean Discharge

Computed from the Continuous Record of Instantaneous

Discharge and the Published Daily Mean Discharge

Appendix E 99

Appendix E. Days with disagreeament greater than or equal to 5 percent between the daily mean discharge computed from the continuous record of instantaneous 

discharge and the published daily mean discharge

[MST, Mountain Standard Time; ft3/s, cubic foot per second; %, percent]

Date

Disagreement

(percent)

Published

discharge

(ft3

/s)

Computed

discharge

(ft3

/s)

Reason for disagreement

6-13-1921 -7.8 153,000 141,000 Peak discharge of the June 1921 flood was revised as part of this study. The published daily 

mean discharges for this period were computed using the 1938 revised rating curve of 

Gatewood and Hunter (rating curve 5-03-38). In this study, discharges prior to June 26, 

1921, were computed using the stage-discharge rating curve in fig. 13.

6-14-1921 -11.8 169,000 149,000

6-15-1921 -15.7 185,000 156,000

6-16-1921 -18.3 197,000 161,000

6-17-1921 -19.1 204,000 165,000

6-18-1921 -21.5 214,000 168,000

6-19-1921 -19.4 201,000 162,000

6-20-1921 -16.4 177,000 148,000

6-21-1921 -12.5 152,000 133,000

6-22-1921 -7.8 129,000 119,000

6-23-1921 -7.0 114,000 106,000

7-26-1921 +8.6 26,600 28,900 The daily mean stage measured on July 26, 1921, was 0.09 ft higher than that measured on July 

25, 1921, and was 0.22 ft higher than that measured on July 27, 1921. The published daily 

mean discharge on July 25, 1921, was 27,700 ft3/s, and the published daily mean discharge 

on July 27, 1921, was 26,600 ft3/s. Therefore, the discharge on July 26, 1921, could not have 

equaled 26,600 ft3/s and should have been slightly higher than 27,700 ft3/s. No discharge 

measurements were made on this day to justify the published value of 26,600 ft3/s.

7-30-1921 +15.3 26,200 30,200 The daily mean stage on July 30, 1921, was 0.59 ft less than that measured on July 29, 1921, 

and 0.52 ft greater than that measured on July 31, 1921. The published daily mean discharge 

on July 29, 1921, was 33,200 ft3/s, and the published daily mean discharge on July 31, 1921, 

was 25,500 ft3/s. Therefore, the daily mean discharge on July 30, 1921, should have been 

closer to 33,200 ft3/s than 25,500 ft3/s. No discharge measurements were made on this day to 

justify the published value of 26,200 ft3/s.

9-15-1921 +5.0 10,000 10,500 On this day, the measured discharge was 10,400 ft3/s and the stage varied by only 0.08 ft. The 

daily mean discharge calculated from the continuous record of instantaneous discharge is 

much closer to the measured value than is the published daily mean discharge.

10-2-1921 -5.9 11,800 11,100 Staff gage readings at 7:35 and 17:40 MST yield a daily mean stage of 8.85 ft for the day. This 

was the stage value used to compute the published daily mean discharge. The real daily 

mean stage was probably lower than 8.85 ft, however, because the reading on October 1, 

1921, at 18:15 was 7.73 ft and the reading on October 3, 1921, at 8:15 was 7.93 ft. In this 

study, linear interpolation was used between the time of all four of these gage readings in the 

computation of the daily mean discharge.

1-13-1922 +8.7 3,900 4,240 Ice

1-14-1922 +22.7 3,700 4,540 Ice

1-15-1922 +16.2 3,700 4,300 Ice

1-16-1922 +9.5 4,110 4,500 Ice

1-17-1922 +7.8 4,110 4,430 Ice

1-29-1922 +9.9 4,430 4,870 Ice
100 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

Appendix E. Days with disagreeament greater than or equal to 5 percent between the daily mean discharge computed from the continuous record of instantaneous 

discharge and the published daily mean discharge—Continued

Date

Disagreement

(percent)

Published

discharge

(ft3

/s)

Computed

discharge

(ft3

/s)

Reason for disagreement

2-03-1922 +11.4 5,020 5,590 Ice

7-11-1922 -5.6 27,000 25,500 In this study, discharge was computed on this day using the stage-discharge rating curve and 

shifts of Gatewood and Hunter. Measured discharge on this day was 25,500 ft3/s, a value in 

agreement with the value computed from the continuous record of instantaneous discharge. 

The published daily mean discharge was computed using a different rating curve without 

shifts.

8-25-1922 -5.7 19,300 18,200 Staff gage readings at 8:25, 11:30, and 19:20 MST yield a daily mean stage of 10.15 ft for the 

day. This was the stage value used to compute the published daily mean discharge. The real 

daily mean stage was probably lower than 10.16 ft, however, because the reading on August 

24, 1922, at 18:55 was 8.92 ft, and the reading on August 26, 1922, at 6:50 was 10.10 ft. In 

this study, linear interpolation was used between the times of these five gage readings to 

compute the daily mean discharge.

6-12-1923 +5.1 70,400 74,200 In this study, discharge was computed on this day using the stage-discharge rating curve and 

shifts of Gatewood and Hunter. The published daily mean discharge was computed using a 

different rating curve without shifts.

6-13-1923 +5.9 69,900 74,000 In this study, discharge was computed on this day using the stage-discharge rating curve and 

shifts of Gatewood and Hunter. Measured discharge on this day was 74,300 ft3/s, a value in 

close agreement with the value computed from the continuous record of instantaneous 

discharge. The published daily mean discharge was computed using a different rating curve 

without shifts.

4-12-1924 +6.0 38,500 40,800 In this study, discharge was computed on this day using the stage-discharge rating curve and 

shifts of Gatewood and Hunter. Measured discharge on this day was 42,200 ft3/s. The daily 

mean discharge of 40,800 ft3/s computed from the continuous record of instantaneous 

discharge is only 3.3% lower than the measured value of 42,200 ft3/s. The published daily 

mean discharge was computed using a different rating curve without shifts.

4-13-1924 +5.2 40,600 42,700 In this study, discharge was computed on this day using the stage-discharge rating curve and 

shifts of Gatewood and Hunter. The published daily mean discharge was computed using a 

different rating curve without shifts.

9-13-1924 +7.9 9,640 10,400 Day should have been subdivided during the computation of the published daily mean 

discharge. Subdivision of the day into four periods would have brought the daily mean 

discharge into close agreement with the value computed from the continuous record of 

instantaneous discharge.

12-24-1924 +12.4 3,400 3,820 Ice

12-25-1924 +14.3 2,800 3,200 Ice

12-26-1924 -13.3 1,500 1,300 Ice

12-28-1924 +13.3 1,200 1,360 Ice

12-30-1924 -11.1 1,350 1,200 Ice

1-01-1925 +23.3 1,500 1,850 Ice

1-02-1925 -5.0 2,400 2,280 Ice

1-24-1925 -5.3 4,900 4,640 Ice

1-28-1925 -6.5 4,900 4,580 Ice

1-31-1925 +6.9 5,800 6,200 Ice

2-01-1925 +7.8 6,000 6,470 Ice
Appendix E 101

2-02-1925 +8.2 6,500 7,030 Ice

2-03-1925 -11.6 8,500 7,510 Ice

2-05-1925 -17.4 8,500 7,020 Ice

1-22-1926 +5.0 5,350 5,620 Ice

1-23-1926 +5.3 5,300 5,580 Ice

1-24-1926 +5.5 5,100 5,380 Ice

12-29-1926 +5.4 2,800 2,950 Ice

12-27-1928 +6.4 3,300 3,510 Ice

8-07-1929 -5.0 65,500 62,200 In this study, discharge was reduced by 3.5% during July 16 through August 26, 1929, on the 

basis of W.E. Dickinson’s December 30, 1929, memorandum (Appendix C). Prior to this 

reduction, the daily mean discharge computed from the continuous record of instantaneous 

discharge was only 2% lower than the published value. See text for justification of this 

reduction.

9-06-1929 -5.6 32,300 30,500 In this study, discharge was reduced by 4.9% during August 26 through September 19, 1929, 

on the basis of W.E. Dickinson’s December 30, 1929, memorandum (Appendix C). 

Prior to this reduction, the daily mean discharges computed from the continuous record of 

instantaneous discharge were within 5% of the published values on these days. See text for 

justification of this reduction.

9-07-1929 -5.7 29,700 28,000

9-08-1929 -6.2 30,800 28,900

9-09-1929 -6.0 40,200 37,800

9-10-1929 -5.6 41,100 38,800

9-11-1929 -5.4 35,400 33,500

9-14-1929 -5.1 25,700 24,400

1-08-1931 -5.6 4,100 3,870 Ice

1-11-1931 -5.7 4,200 3,960 Ice

1-12-1931 -7.1 4,200 3,900 Ice

1-13-1931 -6.1 4,100 3,850 Ice

1-14-1931 -6.0 4,200 3,950 Ice

1-15-1931 -6.8 4,400 4,100 Ice

1-16-1931 -5.6 4,500 4,250 Ice

1-17-1931 -8.7 4,600 4,200 Ice

1-18-1931 -6.4 4,500 4,210 Ice

12-20-1931 +8.3 1,800 1,950 Ice

12-21-1931 +5.6 2,150 2,270 Ice

12-22-1931 +6.3 2,400 2,550 Ice

1-25-1932 +6.9 3,750 4,010 Ice

12-31-1932 -5.0 2,600 2,470 Ice

1-01-1933 -5.9 3,700 3,480 Ice

1-05-1933 -10.8 3,800 3,390 Ice

Appendix E. Days with disagreeament greater than or equal to 5 percent between the daily mean discharge computed from the continuous record of instantaneous 

discharge and the published daily mean discharge—Continued

Date

Disagreement

(percent)

Published

discharge

(ft3

/s)

Computed

discharge

(ft3

/s)

Reason for disagreement
102 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

1-06-1933 +10.9 3,200 3,550 Ice

1-08-1933 +6.1 3,100 3,290 Ice

1-09-1933 +7.5 3,200 3,440 Ice

1-13-1933 -5.3 4,900 4,640 Ice

1-16-1933 -8.9 4,700 4,280 Ice

1-25-1933 -5.4 5,200 4,920 Ice

2-08-1933 +5.5 4,000 4,220 Ice

2-11-1933 -5.0 2,400 2,280 Ice

2-12-1933 -6.2 2,600 2,440 Ice

2-13-1933 -6.2 4,200 3,940 Ice

1-22-1935 +9.4 4,250 4,650 Ice

1-23-1935 +10.0 3,700 4,070 Ice

1-24-1935 +6.8 2,950 3,150 Ice

9-30-1935 -6.0 20,000 18,800 Stage-discharge rating curve was read incorrectly during the computation of the published daily 

mean discharge. Instead of computing the discharge associated with a daily mean stage of 

10.05 ft, the discharge associated with a stage of 10.30 ft was computed.

1-14-1937 +8.0 3,000 3,240 Ice

1-15-1937 -13.6 2,790 2,410 Ice

1-31-1937 -11.1 4,240 3,770 Ice

2-18-1937 +10.2 10,800 11,900 Discharge was copied incorrectly during the computation of the published daily mean 

discharge; 11,800 ft3/s was incorrectly copied as 10,800 ft3/s.

8-28-1937 -5.7 4,240 4,000 Stage-discharge rating curve was read incorrectly during the computation of the published daily 

mean discharge. Instead of computing the discharge associated with a daily mean stage of 

6.40 ft, the discharge associated with a stage of 6.50 ft was computed.

3-15-1940 -5.2 7,140 6,770 Daily mean stage was incorrectly written on the stage-recorder chart as 7.67 ft instead of 7.57 ft 

during the computation of the published daily mean discharge.

8-03-1940 -6.1 3,300 3,100 Daily mean stage was incorrectly computed as 6.10 ft instead of 6.08 ft, and the shift was 

incorrectly applied as +0.05 ft instead of -0.03 ft during the computation of the published 

daily mean discharge.

4-06-1944 +5.7 10,600 11,200 Day was subdivided incorrectly during the district review of the original computations. Day 

was originally subdivided into five periods, yielding a daily mean discharge of 10,950 ft3/s. 

During district review, the day was subdivided into only three periods, yielding a daily mean 

discharge of 10,600 ft3/s. The value of 11,200 ft3/s computed from the continuous record of 

instantaneous discharge is only 2.3% higher than the original value of 10,950 ft3/s.

2-06-1949 -6.0 4,840 4,550 Ice

2-07-1949 -8.0 4,860 4,470 Ice

2-10-1949 -5.1 5,460 5,180 Ice

2-11-1949 -7.2 6,240 5,790 Ice

2-12-1949 -5.2 5,930 5,620 Ice

Appendix E. Days with disagreeament greater than or equal to 5 percent between the daily mean discharge computed from the continuous record of instantaneous 

discharge and the published daily mean discharge—Continued

Date

Disagreement

(percent)

Published

discharge

(ft3

/s)

Computed

discharge

(ft3

/s)

Reason for disagreement
Appendix E 103

1-04-1950 +7.0 5,400 5,780 Ice

1-05-1950 +7.4 5,400 5,800 Ice

1-06-1950 +8.1 5,300 5,730 Ice

1-07-1950 +5.6 5,000 5,280 Ice

1-08-1950 +7.4 4,700 5,050 Ice

11-11-1952 -6.0 6,130 5,760 Daily mean stage was incorrectly written on the stage-recorder chart as 7.29 ft instead of 7.19 ft 

during the computation of the published daily mean discharge. See fig. 19.

1-01-1963 +5.3 2,250 2,370 Ice

1-12-1963 +5.4 2,800 2,950 Ice

1-13-1963 -12.5 1,600 1,400 Ice

1-14-1963 -6.0 3,000 2,820 Ice

1-22-1963 -9.2 1,300 1,180 Ice

1-24-1963 +15.0 700 805 Ice

9-01-1964 -25.6 1,800 1,340 During the computation of the published daily mean discharge, this day was subdivided using 

the wrong mean stage for the hours between 0:00–6:00 MST. The mean stage for these 6 

hours was incorrectly recorded as 6.20 ft instead of 5.20 ft. Use of a 5.20 ft stage for 0:00–

6:00 MST would have resulted in a daily mean discharge of 1,390 ft3/s, a value only 3.7% 

higher than the value computed from the continuous record of instantaneous discharge.

1-21-1965 +8.8 7,250 7,890 Time correction of +100 minutes was not applied during the computation of the published daily 

mean discharge. Application of this time correction would have resulted in a daily mean 

discharge of 8,010 ft3/s, a value only 1.5% higher than the value computed from the 

continuous record of instantaneous discharge.

1-22-1965 -9.3 7,110 6,450 Time correction of +100 minutes was not applied during the computation of the published daily 

mean discharge. Application of this time correction would have resulted in a daily mean 

discharge of 6,370 ft3/s, a value only 1.5% lower than the value computed from the 

continuous record of instantaneous discharge.

3-09-1965 -7.2 6,570 6,100 Published daily mean discharge was computed incorrectly. The mean discharge for the first 12 

hours of the day was computed as 5,090 ft3/s, and the mean discharge for the second 12 

hours of the day was computed as 7,050 ft3/s. The average of these two numbers is 6,070 

ft3/s. Instead, a value of 6,570 ft3/s was written in the final computations.

4-22-1965 +5.6 12,500 13,200 Time correction of -45 minutes was not applied during the computation of the published daily 

mean discharge. Application of this time correction would have resulted in a daily mean 

discharge of 13,500 ft3/s, a value only 2.3% higher than the value computed from the 

continuous record of instantaneous discharge.

12-03-1965 +6.5 9,450 8,840 Day was subdivided incorrectly during the computation of the published daily mean discharge. 

Day was only subdivided into three periods, resulting in the published daily mean discharge 

of 9,450 ft3/s. Subdivision of the day into five periods would have brought the daily mean 

discharge into close agreement with the value computed from the continuous record of 

instantaneous discharge.

12-05-1965 +6.0 7,150 6,720 Day should have been subdivided during the computation of the published daily mean 

discharge.

Appendix E. Days with disagreeament greater than or equal to 5 percent between the daily mean discharge computed from the continuous record of instantaneous 

discharge and the published daily mean discharge—Continued

Date

Disagreement

(percent)

Published

discharge

(ft3

/s)

Computed

discharge

(ft3

/s)

Reason for disagreement
104 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

12-31-1965 -5.2 6,500 6,160 During the computation of the published daily mean discharge, this day was subdivided using 

the wrong mean stage for the hours between 13:00–24:00 MST. The mean stage for these 

last 11 hours of the day was recorded as 7.05 ft when it was actually closer to 7.80 ft. Use of 

a 7.80 ft stage for the last 11 hours of the day would have resulted in a daily mean discharge 

of 6,200 ft3/s, a value only 0.6% lower than the value computed from the continuous record 

of instantaneous discharge.

1-26-1966 -11.4 12,300 10,900 During the computation of the published daily mean discharge, this day was subdivided using 

the wrong mean discharge for the hours between 3:00–10:00 MST. The mean stage for these 

7 hours was written as 7.37 ft on the stage-recorder chart. This corresponded to a discharge 

of 5,790 ft3/s on the stage-discharge rating curve in use on January 26, 1966. The incorrect 

discharge used for these 7 hours was 11,200 ft3/s during the original computation. Use of the 

correct mean discharge for 3:00–10:00 MST would have resulted in a daily mean discharge 

of 10,800 ft3/s, a value only 0.9% lower than the value computed from the continuous record 

of instantaneous discharge.

8-07-1966 -7.6 10,600 9,790 During the computation of the published daily mean discharge, this day was subdivided using 

the wrong mean stage for the hours between 6:00–12:00 MST. The mean stage for these 6 

hours written on the stage-recorder chart was 8.16 ft, instead of the correct value of 7.16 ft. 

Use of a 7.16 ft stage for these 6 hours would have resulted in a daily mean discharge of 

9,500 ft3/s, a value only 3.0% lower than the value computed from the continuous record of 

instantaneous discharge.

10-09-1966 -9.6 8,120 7,340 During the computation of the published daily mean discharge, a mathematical error was made 

in the averaging process. The average of the mean discharges for the four periods into which 

the day was subdivided is 7,310 ft3/s, not the 8,120 ft3/s value written on the stage-recorder 

chart.

10-24-1966 +5.8 8,340 8,820 During the computation of the published daily mean discharge, this day was subdivided 

incorrectly (into only three periods). Subdivision of the day into four periods would have 

resulted in a daily mean discharge of 8,500 ft3/s, a value only 3.6% lower than the value 

computed from the continuous record of instantaneous discharge.

10-31-1966 +7.8 10,300 11,100 During the computation of the published daily mean discharge, this day was subdivided using 

the wrong mean stage for the hours between 3:00–12:00 MST. The mean stage for these 9 

hours written on the stage-recorder chart was 6.88 ft, when, according to the chart, it was 

closer to 7.13 ft. Use of a 7.13 ft stage for these 9 hours would have resulted in a daily mean 

discharge of 11,000 ft3/s, a value only 0.9% lower than the value computed from the 

continuous record of instantaneous discharge.

11-06-1966 +9.1 4,600 5,020 During the computation of the published daily mean discharge, this day was subdivided using 

the wrong mean stage for the hours between 0:00–12:00 MST. The mean stage for these 6 

hours written on the stage-recorder chart was 6.79 ft, when, according to the chart, it was 

closer to 7.05 ft. Use of a 7.05 ft stage for these 12 hours would have resulted in a daily mean 

discharge of 4,930 ft3/s, a value only 1.8% lower than the value computed from the 

continuous record of instantaneous discharge.

1-27-1967 +6.6 9,850 10,500 During the computation of the published daily mean discharge, this day was subdivided 

incorrectly (into only three periods). Subdivision of the day into four periods would have 

resulted in a daily mean discharge of 10,500 ft3/s, a value in perfect agreement with the value 

computed from the continuous record of instantaneous discharge.

Appendix E. Days with disagreeament greater than or equal to 5 percent between the daily mean discharge computed from the continuous record of instantaneous 

discharge and the published daily mean discharge—Continued

Date

Disagreement

(percent)

Published

discharge

(ft3

/s)

Computed

discharge

(ft3

/s)

Reason for disagreement
Appendix E 105

2-02-1967 -11.5 12,200 10,800 During the computation of the published daily mean discharge, this day was subdivided using 

the wrong mean stage for the hours between 3:00–12:00 MST. The mean stage for these 9 

hours written on the stage-recorder chart was 8.92 ft, when, according to the chart, it was 

closer to 8.00 ft. Use of a 8.00 ft stage for these 9 hours would have resulted in a daily mean 

discharge of 10,800 ft3/s, a value in perfect agreement with the value computed from the 

continuous record of instantaneous discharge.

6-01-1967 -5.5 10,900 10,300 Published daily mean discharge was copied incorrectly from the final computations. Daily 

mean discharge written in the final computations in the annual technical file was 10,100 ft3/s.

11-19-1967 -16.4 5,420 4,530 During the computation of the published daily mean discharge, the daily mean discharge was 

copied incorrectly from the stage-recorder chart. Daily mean discharge written on the stagerecorder chart was 4,520 ft3/s; daily mean discharge copied into the final computations was 

5,420 ft3/s. The 5 and the 4 were transposed when the number was copied.

1-24-1970 +6.1 4,590 4,870 Lower intakes in the gage stilling well were plugged on January 24 and 25, 1970. During the 

computation of the published daily mean discharge, these days were subdivided using 

estimated mean stages that were probably too low for parts of each day.

1-25-1970 +5.1 2,530 2,660

2-08-1970 +7.7 3,100 3,340 Lower intakes in the gage stilling well were plugged. During the computation of the published 

daily mean discharge, the day was subdivided using an estimated mean stage for the hours of 

4:00–21:00 MST that was probably too low.

3-11-1971 -7.8 11,600 10,700 Error of unknown origin in the published daily mean discharges for March 11 and 12, 1971. 

Published record for March 10, 1971, through March 13, 1971, has too much water passing 

Lees Ferry relative to the gaging station immediately below Glen Canyon Dam. Mean of the 

published discharges at the gaging station below Glen Canyon Dam for March 9, 1971, 

through March 12, 1971, is 10,200 ft3/s, mean of the published discharges at Lees Ferry for 

March 10, 1971, through March 13, 1971, is 10,800 ft3/s. In this study, instantaneous 

discharges were computed using stages from the time-corrected stage-recorder graph record 

on these days. The mean discharge for March 10, 1971, through March 13, 1971, computed 

from the continuous record of instantaneous discharge is 10,400 ft3/s (a value only 2.0% 

higher than 10,200 ft3/s).

3-12-1971 -6.3 11,200 10,500

11-07-1971 +5.3 7,560 7,960 Published daily mean discharge on this day was based on stages digitally recorded every 2 

hours. The discharge on this day, however, was more variable than could be captured by the 

2-hour punch data. Daily mean stage determined from the stage-recorder graph was 7.75 ft; 

daily mean stage determined from the bi-hourly digital stage record was 7.69 ft. This stage 

difference corresponds to a discharge error of about -460 ft3/s.

12-16-1973 +5.4 3,330 3,510 Published daily mean discharge on this day was based on stages digitally recorded every 2 

hours. The discharge on this day, however, was more variable than could be captured by the 

2-hour punch data. Daily mean stage determined from stage-recorder graph was 6.14 ft; 

daily mean stage determined from the bi-hourly digital stage record was 6.07 ft. This stage 

difference corresponds to a discharge error of about -140 ft3/s.

3-01-1974 -22.4 11,000 8,540 Published daily mean discharge was copied incorrectly from the final computations. The daily 

mean discharge written in the final computations in the annual technical final was 8,400 ft3/s.

Appendix E. Days with disagreeament greater than or equal to 5 percent between the daily mean discharge computed from the continuous record of instantaneous 

discharge and the published daily mean discharge—Continued

Date

Disagreement

(percent)

Published

discharge

(ft3

/s)

Computed

discharge

(ft3

/s)

Reason for disagreement
106 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

4-23-1975 +5.1 8,750 9,200 Digital recorder error. Digital stage recorder was erratic the entire month of April 1975. Values 

recorded on the digital recorder on this day were especially erratic. Because the stages 

recorded on the stage-recorder graph seemed more correct, these stages were used in this 

study to compute discharges on this day.

12-26-1976 +5.1 4,530 4,760 Gage was silted in on this day. During the computation of the published daily mean discharge, 

this day was subdivided by hand using the wrong mean stage for the hours between 14:00–

21:00 MST. The mean stage for these 7 hours written on the chart was 6.80 ft, when, 

according to the chart, it was closer to 6.95 ft. Use of a 6.95 ft stage for these 7 hours would 

have resulted in a daily mean discharge of 4,640 ft3/s, a value only 2.5% lower than the value 

computed from the continuous record of instantaneous discharge.

3-08-1977 +10.5 7,320 8,090 Published daily mean discharge was determined by subdividing the record on the stagerecorder chart instead of using the digital record. Daily mean discharge from the digital 

recorder was 8,110 ft3/s, only 0.2% greater than our value of 8,090 ft3/s. During hand 

subdivision, the wrong mean stage was used for the hours between 3:00–12:00 MST. The 

mean stage for these 9 hours written on the chart was 5.55 ft, when, according to the chart, it 

was closer to 6.55 ft. Use of a 6.55 ft stage for these 9 hours would have resulted in a daily 

mean discharge of 8,010 ft3/s, a value only 1.0% lower than the value computed from the 

continuous record of instantaneous discharge.

3-30-1977 +5.1 8,580 9,020 Published daily mean discharge was computed from the digital record mistakenly using only 

the stages measured from 16:00–24:00 MST.

7-31-1977 +37.6 12,500 17,200 Digital record for this day was lost by the USGS. Based on the stage-recorder chart record, the 

daily mean discharge computed from the continuous record of instantaneous discharge 

seems to be correct. The stage-recorder chart record indicates that daily mean discharge for 

this day should be close to that on July 30, 1977 (i.e., 17,100 ft3/s).

11-24-1977 -6.0 3,150 2,960 The published daily mean discharge was computed from the stage-recorder chart record. 

During hand subdivision of the record, a mean stage of 5.50 ft was used for the hours of 

6:00–12:00 MST, when the mean stage for these 6 hours was closer to 5.75 ft.

11-29-1977 -14.0 6,410 5,510 Published daily mean discharge was based only on the digital record for the hours of 14:00–

24:00 MST. Daily mean discharge using the stage-recorder chart record for 0:00–14:00 

MST and the digital record for 14:00–24:00 MST was written on the printout of the digital 

record as 5,390 ft3/s. This value was not copied into the final computations.

3-07-1979 +12.7 1,100 1,240 Gage was silted in during many hours on March 7 through 25, 1979. The published daily mean 

discharges for this period were thus determined by hand subdivision of the stage-recorder 

chart record. During this process, time corrections of 1–2 hours were not applied, leading to 

the errors in the published record.

3-08-1979 -10.3 1,260 1,130

3-11-1979 -16.8 1,170 973

3-12-1979 +12.6 1,030 1,160

3-18-1979 +17.3 878 1,030

3-25-1979 -7.0 1,150 1,070

Appendix E. Days with disagreeament greater than or equal to 5 percent between the daily mean discharge computed from the continuous record of instantaneous 

discharge and the published daily mean discharge—Continued

Date

Disagreement

(percent)

Published

discharge

(ft3

/s)

Computed

discharge

(ft3

/s)

Reason for disagreement
Appendix E 107

12-27-1979 +10.5 11,400 12,600 Digital recorder malfunction or digital tape read incorrectly during the computation of the 

published daily mean discharge. Digital record abruptly lost 13.5 hours between 13:00 and 

16:00 MST on December 27, 1979. Digital record from 16:00 MST on December 27, 1979. 

through 13:00 MST on January 3, 1980, was 13.5 hours behind the stage-recorder chart 

record. Digital record then abruptly regained the 13.5 hours it lost at 13:00 MST on January 

3, 1980. Based on comparison with the record from the Grand Canyon gaging station, the 

stage-recorder chart record for this period was probably correct. In this study, therefore, 

discharges for this period were computed by using the time-corrected stage-recorder chart 

record.

12-29-1979 +80.4 4,790 8,640

12-30-1979 -14.2 2,530 2,170

12-31-1979 +64.3 2,130 3,500

1-01-1980 -48.7 2,650 1,360

1-02-1980 -26.1 8,560 6,330

10-09-1980 +7.2 15,300 16,400 Published daily mean discharge was computed using the stage-recorder graph record. Values 

written on the chart using two different subdivisions were 16,000 and 16,400 ft3/s. These 

values were copied incorrectly into the final computations as 15,300 ft3/s.

3-26-1982 +9.8 12,200 13,400 Digital recorder malfunction between 19:00–21:00 MST. Stage value recorded at 19:00 was 

6.53 ft, when it should have been 9.53 ft; stage value recorded at 20:00 was 6.74 ft when it 

should have been 9.74 ft; stage value recorded at 21:00 was 7.36 ft, when it should have been 

10.06 ft.

7-09-1982 +27.6 12,300 15,700 Published daily mean discharge was computed by hand subdivision of the stage-recorder chart 

record. During hand subdivision, the wrong mean stage was used for the hours between 

12:00–24:00 MST. The mean stage for these 12 hours was written as 9.5 ft, when, according 

to the chart, it was closer to 10.5 ft.

3-02-1983 +22.5 12,900 15,800 Digital recorder malfunction from 13:00 MST on March 2, 1983, through 5:00 MST on March 

3, 1980. Stage values in the digital record between these times were determined incorrectly 

by linearly interpolation.

3-03-1983 +5.3 13,200 13,900

4-10-1983 -6.2 16,100 15,100 Digital recorder malfunction. Published daily mean discharge was computed by hand using an 

incorrect weighted mean stage of 9.87 ft for the day. Daily mean stage was 9.67 ft, weighted 

daily mean stage should have been closer to 9.7 ft.

6-03-1986 -8.5 44,500 40,700 Digital recorder malfunction from 1:00-13:00 MST. Digital stage record is 1.13 ft too high 

between 5:00 and 9:00 MST. Stages in the digital record were linearly interpolated between 

the correct value at 1:00 MST and the incorrect value at 5:00 MST, and between the 

incorrect value at 9:00 MST and the correct value at 13:00 MST.

Appendix E. Days with disagreeament greater than or equal to 5 percent between the daily mean discharge computed from the continuous record of instantaneous 

discharge and the published daily mean discharge—Continued

Date

Disagreement

(percent)

Published

discharge

(ft3

/s)

Computed

discharge

(ft3

/s)

Reason for disagreement

APPENDIX F: Monthly Flow-Duration Curves

Computed from the Continuous Record of

Instantaneous Discharge

Appendix F 111

0 10 20 30 40 50 60 70 80 90 100

100

1,000

10,000

100,000

1,000,000

PRE-DAM 

POST-DAM 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

INSTANTANEOUS DISCHARGE, IN CUBIC FEET PER SECOND

SAND

CONVEYANCE

OR

EROSION

SAND

ACCUMULATION

1.1%

75.2%

JANUARY

MEDIAN VALUE

0 10 20 30 40 50 60 70 80 90 100

100

1,000

10,000

100,000

1,000,000

PRE-DAM 

POST-DAM 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

INSTANTANEOUS DISCHARGE, IN CUBIC FEET PER SECOND

SAND

CONVEYANCE

OR

EROSION

SAND

ACCUMULATION

9.4%

67.8%

FEBRUARY

MEDIAN VALUE

0 10 20 30 40 50 60 70 80 90 100

100

1,000

10,000

100,000

1,000,000

PRE-DAM

POST-DAM 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

INSTANTANEOUS DISCHARGE, IN CUBIC FEET PER SECOND

SAND

CONVEYANCE

OR

EROSION

SAND

ACCUMULATION

36.8%

60.4%

MARCH

MEDIAN VALUE

0 10 20 30 40 50 60 70 80 90 100

100

1,000

10,000

100,000

1,000,000

PRE-DAM

POST-DAM 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

INSTANTANEOUS DISCHARGE, IN CUBIC FEET PER SECOND

SAND CONVEYANCE

OR

EROSION

SAND

ACCUMULATION

79.9%

69.4%

APRIL

MEDIAN VALUE

A.

C. D.

B.

Figure F1. The monthly flow-duration curves for the pre- and post-dam periods of record for the Colorado River at Lees Ferry. The gray shaded region 

shows the pre-dam discharge range under which sand accumulated in the reach between the Lees Ferry and Grand Canyon gaging stations, in Marble and 

upper Grand Canyons (Topping and others, 2000). The percentages of time each month when these discharges were exceeded during the pre- and postdam periods are indicated in italics. (A) January. (B) February. (C) March. (D) April.
112 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

0 10 20 30 40 50 60 70 80 90 100

100

1,000

10,000

100,000

1,000,000

PRE-DAM

POST-DAM 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

INSTANTANEOUS DISCHARGE, IN CUBIC FEET PER SECOND

SAND

CONVEYANCE

OR

EROSION

SAND

ACCUMULATION

99.6%

69.2%

MAY

MEDIAN VALUE

0 10 20 30 40 50 60 70 80 90 100 100

1,000

10,000

100,000

1,000,000

PRE-DAM

POST-DAM 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

INSTANTANEOUS DISCHARGE, IN CUBIC FEET PER SECOND

SAND

CONVEYANCE OR

EROSION

SAND

ACCUMULATION

98.6%

70.7%

JUNE

MEDIAN VALUE

E. F.

0 10 20 30 40 50 60 70 80 90 100

100

1,000

10,000

100,000

1,000,000

PRE-DAM

POST-DAM 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

INSTANTANEOUS DISCHARGE, IN CUBIC FEET PER SECOND

SAND

CONVEYANCE

OR EROSION

SAND

ACCUMULATION

75.5% 82.0%

JULY

MEDIAN VALUE

0 10 20 30 40 50 60 70 80 90 100 100

1,000

10,000

100,000

1,000,000

PRE-DAM

POST-DAM 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

INSTANTANEOUS DISCHARGE, IN CUBIC FEET PER SECOND

SAND

CONVEYANCE

OR

EROSION

SAND

ACCUMULATION

78.2%

47.0%

AUGUST

MEDIAN VALUE

G. H.

Figure F1—Continued. Monthly flow-duration curves for the pre- and post-dam periods of record for the Colorado River at Lees Ferry. The gray shaded 

region shows the pre-dam discharge range under which sand accumulated in the reach between the Lees Ferry and Grand Canyon gaging stations, in 

Marble and upper Grand Canyons (Topping and others, 2000). The percentages of time each month when these discharges were exceeded during the preand post-dam periods are indicated in italics. (E) May. During May in the pre-dam period, discharges under which sand accumulated in Marble and upper 

Grand Canyons were exceeded 99.6 percent of the time. May was the pre-dam month of greatest erosion in Marble and upper Grand Canyons (Topping 

and others, 2000, fig. 10C). (F) June. During this month in the pre-dam period, discharges under which sand accumulated in Marble and upper Grand 

Canyons were exceeded 98.6 percent of the time. June was the pre-dam month of second greatest erosion in Marble and upper Grand Canyons (Topping 

and others, 2000, fig. 10C). (G) July. During this month in the pre-dam period, discharges under which sand could likely accumulate in Marble and upper 

Grand Canyons were exceeded 75.5 percent of the time; during July in the post-dam period, these discharges were exceeded 82.0 percent of the time. 

Although this suggests that sand should have been eroded, minor amounts of sand did accumulate in Marble and upper Grand Canyons during both the 

pre- and post-dam months of July because of the increased sediment supply from tributaries during this month (Topping and others, 2000, fig. 10). 

(H) August. Because this month is the month of greatest tributary sediment supply, and was characterized by low to moderate discharges during the predam period, August was the pre-dam month of greatest sediment accumulation in Marble and upper Grand Canyons (Topping and others, 2000, fig. 10). 
Appendix F 113

0 10 20 30 40 50 60 70 80 90 100

100

1,000

10,000

100,000

1,000,000

PRE-DAM

POST-DAM 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

INSTANTANEOUS DISCHARGE, IN CUBIC FEET PER SECOND

SAND

CONVEYANCE

OR

EROSION

SAND

ACCUMULATION

71.4%

27.7%

SEPTEMBER

MEDIAN VALUE

0 10 20 30 40 50 60 70 80 90 100

100

1,000

10,000

100,000

1,000,000

PRE-DAM

POST-DAM 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

INSTANTANEOUS DISCHARGE, IN CUBIC FEET PER SECOND

SAND

CONVEYANCE

OR

EROSION

SAND

ACCUMULATION

56.9%

27.0%

OCTOBER

MEDIAN VALUE

0 10 20 30 40 50 60 70 80 90 100

100

1,000

10,000

100,000

1,000,000

PRE-DAM

POST-DAM 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

INSTANTANEOUS DISCHARGE, IN CUBIC FEET PER SECOND

SAND

CONVEYANCE OR

EROSION

SAND

ACCUMULATION

63.7% 18.2%

NOVEMBER

MEDIAN VALUE

0 10 20 30 40 50 60 70 80 90 100

100

1,000

10,000

100,000

1,000,000

PRE-DAM

POST-DAM 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

INSTANTANEOUS DISCHARGE, IN CUBIC FEET PER SECOND

SAND

CONVEYANCE

OR

EROSION

SAND

ACCUMULATION

 69.7% 4.4%

DECEMBER

MEDIAN VALUE

K. L.

J. I.

Figure F1—Continued. Monthly flow-duration curves for the pre- and post-dam periods of record for the Colorado River at Lees Ferry. The gray shaded 

region shows the pre-dam discharge range under which sand accumulated in the reach between the Lees Ferry and Grand Canyon gaging stations, in 

Marble and upper Grand Canyons (Topping and others, 2000). The percentages of time each month when these discharges were exceeded during the preand post-dam periods are indicated in italics. (I) September. (J) October. (K) November (L) December. 

APPENDIX G: Monthly Exceedance Curves of the

Daily Range in Discharge Computed from the

Continuous Record of Instantaneous Discharge

Appendix G 117

0 10 20 30 40 50 60 70 80 90 100

10

100

1,000

10,000

100,000

PRE-DAM

POST-DAM 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

DAILY RANGE IN DISCHARGE, IN CUBIC FEET PER SECOND

JANUARY

MEDIAN VALUE

0 10 20 30 40 50 60 70 80 90 100

10

100

1,000

10,000

100,000

PRE-DAM

POST-DAM 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

DAILY RANGE IN DISCHARGE, IN CUBIC FEET PER SECOND

FEBRUARY

MEDIAN VALUE

0 10 20 30 40 50 60 70 80 90 100

10

100

1,000

10,000

100,000

PRE-DAM

POST-DAM

PERCENTAGE OF TIME EQUALED OR EXCEEDED

DAILY RANGE IN DISCHARGE, IN CUBIC FEET PER SECOND

MARCH

MEDIAN VALUE

0 10 20 30 40 50 60 70 80 90 100

10

100

1,000

10,000

100,000

PRE-DAM

POST-DAM 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

DAILY RANGE IN DISCHARGE, IN CUBIC FEET PER SECOND

APRIL

MEDIAN VALUE

0 10 20 30 40 50 60 70 80 90 100

10

100

1,000

10,000

100,000

PRE-DAM 

POST-DAM 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

DAILY RANGE IN DISCHARGE, IN CUBIC FEET PER SECOND

MAY

MEDIAN VALUE

0 10 20 30 40 50 60 70 80 90 100

10

100

1,000

10,000

100,000

PRE-DAM

POST-DAM 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

DAILY RANGE IN DISCHARGE, IN CUBIC FEET PER SECOND

JUNE

MEDIAN VALUE

A.

E. F.

C. D.

B.

Figure G1. Monthly exceedance curves of the daily range in the discharge of the Colorado River at Lees Ferry for the pre- and post-dam periods. 

(A) January. (B) February. (C) March. (D) April. (E) May. (F) June. 
118 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 2000

0 10 20 30 40 50 60 70 80 90 100

10

100

1,000

10,000

100,000

PRE-DAM

POST-DAM 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

DAILY RANGE IN DISCHARGE, IN CUBIC FEET PER SECOND

JULY

MEDIAN VALUE

0 10 20 30 40 50 60 70 80 90 100

10

100

1,000

10,000

100,000

PRE-DAM

POST-DAM 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

DAILY RANGE IN DISCHARGE, IN CUBIC FEET PER SECOND

AUGUST

MEDIAN VALUE

0 10 20 30 40 50 60 70 80 90 100

10

100

1,000

10,000

100,000

PRE-DAM

POST-DAM 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

DAILY RANGE IN DISCHARGE, IN CUBIC FEET PER SECOND

SEPTEMBER

MEDIAN VALUE

0 10 20 30 40 50 60 70 80 90 100

10

100

1,000

10,000

100,000

PRE-DAM

POST-DAM 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

DAILY RANGE IN DISCHARGE, IN CUBIC FEET PER SECOND

OCTOBER

MEDIAN VALUE

0 10 20 30 40 50 60 70 80 90 100

10

100

1,000

10,000

100,000

PRE-DAM 

POST-DAM 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

DAILY RANGE IN DISCHARGE, IN CUBIC FEET PER SECOND

NOVEMBER

MEDIAN VALUE

0 10 20 30 40 50 60 70 80 90 100

10

100

1,000

10,000

100,000

PRE-DAM

POST-DAM 

PERCENTAGE OF TIME EQUALED OR EXCEEDED

DAILY RANGE IN DISCHARGE, IN CUBIC FEET PER SECOND

DECEMBER

MEDIAN VALUE

G.

K. L.

I. J.

H.

Figure G1—Continued. Monthly exceedance curves of the daily range in the discharge of the Colorado River at Lees Ferry for the pre- and 

post-dam periods. (G) July. (H) August. (I) September. (J) October. (K) November. (L) December.