Floods in Utah, * Magnitude and Frequency GEOLOGICAL SURVEY CIRCULAR 457 Floods in Utah, Magnitude and Frequency By V. K. Berwick GEOLOGICAL SURVEY CIRCULAR 457 Washington 1962 United States Department of the Interior STEWART L. UDALL, SECRETARY Geological Survey THOMAS B. NOLAN, DIRECTOR Free on application to the U.S. Geological Survey, Washington 25, D. C. CONTENTS Page Page Abstract >................-......... 1 Flood-frequency relations Continued Introduction. __-_----__------_----_-. 1 Regional flood frequency Continued Cooperation and supervision__________ 1 Composite frequency curve _______ 13 Description of the area___-_-_____._-- 1 Regional analysis for summer Physical features.__-------_----_-. 2 floods _______________________ 13 Colorado River basin ____________ 2 Derivation of the mean annual flood._ 15 The Great Basin...._____________ 2 Basin characteristics .___._._____ 15 Snake River basin __-_-__-___-___ 3 Mean annual flood relations.______ 15 Major rivers_______ _____--._-_ 3 Major rivers in Utah.._____________ 18 Climatic features._________________ 3 Cloudburst floods and mud-rock Flood - frequency relations ____________ 3 flows. ...-.__________________ Flood frequency at a gaging station __ 4 Application of flood-frequency Basic data ______________________ 4 curves ________________________ Flood series ____________________ 4 Method _________________________ Station frequency curve __________ 4 Use of flood-frequency analysis on Regional flood frequency......______ 4 major river s__ _______ ______ ._ Regional sampling............... 13 Limitations _______________________ Selection of comparable floods ____ 13 Homogeneity ____________________ 13 ILLUSTRATIONS Page Figure 1. Map of Utah showing location of gaging stations _____________________________ 5 2. Map of Utah showing flood regions.....____________________________________ 12 3. Composite flood-frequency curves, regions A and B _________________________ 14 4. Composite flood-frequency curves, regions C and D _________________________ 14 5. Map of Utah showing hydrologic areas__-_______.___________________________ 16 6. Variation of mean annual flood with drainage area and mean altitude.__________ 17 7. Colorado River, discharge of 50-year flood____________________________---__ 18 8. Green River, discharge of 50-year flood____________________________________ 18 9. Duchesne River, discharge of 50-year flood.________________________________ 18 10. Strawberry River, discharge of 50-year flood__________________________-_--_ 18 11. Price River, discharge of 50-year flood___-___-______----_---_-------_--_-_ 18 12. San Rafael River, discharge of 50-year flood.....___________________________ 19 13. San Juan River, discharge of 50-year flood _________________________________ 19 14. Bear River, discharge of 50-year flood. ____________________________________ 19 15. Weber River, discharge of 50-year flood ___________________________________ 19 16. Provo River, discharge of 50-year flood________.___________________________ 19 17. Sevier River, discharge of 50-year flood ___________________________________ 19 TABLES Page Table 1. Peak discharges at gaging stations used in frequency analysis 6 2. Main stem gaging-station data_____________________________ 20 HI Floods in Utah, Magnitude and Frequency By V. K. Berwick ABSTRACT records adjusted to a common time base is a This report presents a procedure for estimating the mag­ logical means for predicting future flood ex­ nitude and frequency of floods, within the range of the base pectancy anywhere within a homogeneous flood data, for any site, gaged or ungaged. From the relation of an­ region. This report (a) outlines the homoge­ nual floods to the mean annual flood, a composite frequency curve was derived for recurrence intervals of 1.1 to 50 years. neous flood regions in Utah and presents a For regions of similar hydrologic characteristics, curves were composite flood-frequency curve for each, and developed by multiple correlation to express the relation of (b) outlines hydrologic areas and presents mean aunual flood to drainage area and mean altitude. The record^ of gaging stations having 5 or more years of record graphs .showing the variation of mean annual were used as base data when the natural conditions of stream- flood with drainage area and altitude. flow are not affected by works of man. For major rivers where the flow is affected by diversion or regulation, separate analyses were made for each stream. The results may be applied to any area in Utah, except the Great Salt Lake COOPERATION AND SUPERVISION Desert and a small area of the State in the Snake River basin. This report was prepared by the Geological INTRODUCTION Survey, in cooperation with the Utah Depart­ ment of Highways and the U.S. Bureau of The proper design of structures in or near Public Roads. It was prepared in the office a stream should include the determination of of the Geological Survey at Salt Lake City, the magnitude and frequency of floods. Where Utah, under the direction of M. T. Wilson, flooding may endanger human life, structures district engineer. Technical guidance was should be designed to withstand the greatest furnished by the Floods Section, Water Re­ probable flood magnitude. In most instances, sources Division, Geological Survey, Wash­ however, the risk to human life from flooding ington, D. C. is small, and structures are designed to with­ stand a flood of some selected frequency of DESCRIPTION OF THE AREA occurrence based on economics. High, rugged mountain ranges are the most Floods result from the combined effects of distinguishing features of the Utah landscape. climatic events and physiographic charac­ Their presence influences the flow of air teristics of a basin. Some of the principal masses and therefore has a major affect upon physiographic factors affecting flood flows precipitation and resulting streamflow. The are: drainage area, altitude, geology, basin State is divided by the Wasatch Mountains, a shape, slope, aspect, and vegetal cover. high range that runs in a north-south direc­ tion. On both sides of this divide are extensive Flood data for an individual site show what deserts or arid regions with the exception of has occurred at that site during a definite a humid section in the northern mountain period of time but could lead to erroneous ranges. The Uinta Mountains, one of the few results in the prediction of future events, even mountain ranges in the Unites States with an at that site, if the record is not representative east-west axis, extend eastward from the of the long-term average. A composite flood- north-central part of the State almost to the frequency curve based on many gaging-station Utah-Colorado State line. FLOODS IN UTAH, MAGNITUDE AND FREQUENCY The absence of a usable water supply in the The geology of the Great Basin is complex arid regions of Utah has discouraged the col­ and is not discussed here except in general lection of enough streamflow records to de­ terms. The Wasatch Mountains are charac­ fine flood-frequency relations; therefore, the terized by rugged topography with high peaks Great Salt Lake Desert has not been included and deep canyons. The mountains are long in this analysis. narrow ridges which rise abruptly from the valley floors with relatively small foothill areas. Several streams, including the Bear, PHYSICAL FEATURES Weber, and Provo Rivers, are classed as relatively old and have cut through the high COLORADO RIVER BASIN mountain ranges in deep canyons. Many of the smaller streams in the area are younger The Colorado River basin within Utah con­ in origin, and for the most part are respon­ tains approximately 41,000 square miles, with sible for much of the more recent erosion. plateaus and mountains ranging in altitude from 3,000 to 13,000 feet above sea level. The High Plateaus consist of three parallel The Uinta Mountains bisect the Green River strips of tabletop formations that are com­ basin near the Utah-Wyoming State line, mon to both the Great Basin and the Colorado forming the northern boundary of the Utah River basin. The altitudes range from 5,000 portion of the basin. The Wasatch Mountains to 10,000 feet above sea level. The streams and High Plateaus form the western boundary. originating in the High Plateaus have steep Other mountain ranges and plateaus are in­ slopes leading to gently sloping valley floors. terspersed throughout the area. Some of the When a stream reaches the mouth of a can­ more prominent ranges are the LaSal, Abajo, yon, where the gradient is lower, the flow and Henry Mountains. Another orographic often spreads out on the valley floor. In some barrier in the Colorado River basin, known places, the surface flow disappears as the as the Book Cliffs, extends in a general east- streams cross the alluvial fans at the en­ west direction near the central part of the trance to mountain valleys. These conditions State. Drainage is predominately by streams are more prevalent in the Great Basin, but with deep canyons and relatively steep'slopes. apply to some degree in the Colorado River Drainage areas of streams used in the study basin. for the Colorado River basin range from 2 to 77,000 square miles. The vegetal cover in the Great Basin is similar to that in the Colorado River basin; The vegetal cover within the region varies however, in the northern part of both basins, considerably in kind and in growth depending the average annual precipitation is higher, largely upon the amount and seasonal distri­ and at some altitudes the forest