Prepared in cooperation with the Alaska Department of Fish and Game and City and Borough of Juneau Hydrology, Geomorphology, and Flood Profiles of the Mendenhall River, Juneau, Alaska Water-Resources Investigations Report 99-4150 U.S. Department of the Interior U.S. Geological Survey Cover: Surveying flood marks on the Mendenhall River near cross section 74 Hydrology, Geomorphology, and Flood Profiles of the Mendenhall River, Juneau, Alaska By Edward G. Neal and Randy H. Host __________________________________________________________________ U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 99-4150 Prepared in cooperation with the ALASKA DEPARTMENT OF FISH AND GAME and CITY AND BOROUGH OF JUNEAU Anchorage, Alaska 1999 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Charles G. Groat, Director Use of trade names in this report is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey. For additional information: Copies of this report may be purchased from: District Chief U.S. Geological Survey U.S. Geological Survey Branch of Information Services 4230 University Drive, Suite 201 Box 25286 Anchorage, AK 99508-4664 Denver, CO 80225-0286 http://ak.water.usgs.gov CONTENTS Abstract . .1 Introduction . .1 Purpose and Scope . .2 Description of Study Area . .2 Hydrology of the Mendenhall River Basin . .4 Geomorphology of the Mendenhall River . .4 Mendenhall River Channel Formation . .4 Present Channel Conditions . .5 Regional Uplift . .13 Mendenhall River Channel Adjustments . .14 Methods of Analysis . .15 Hydrologic Analysis . .15 Hydraulic Analysis . .16 Peak Flow of October 20, 1998 . .16 Flood Profile Computations . .19 Flood Profiles . .20 Summary . .21 References Cited . .34 Appendix: Elevation Reference Marks . .35 FIGURES 1. Map showing location of Mendenhall River in Juneau, Alaska. .3 2. Photographs showing aerial views of Mendenhall River from Mendenhall Lake outlet to mouth of Fritz Cove, and locations of selected cross sections and reference marks. .6 3. Photograph of upright stumps and root structures in channel bed, Mendenhall River, Alaska . .13 4. Cross section 74, showing typical channel conditions in the lower reach of the Mendenhall River, Alaska . .17 5. Cross section 93, showing upstream side of the Mendenhall River Road Bridge, Alaska . .18 6. Profile of computed water-surface elevation for the peak flow of October 20, 1998, minimum streambed elevations, location of cross sections, and observed floodmarks, Mendenhall River, Alaska. .19 7-11. Profiles of computed water-surface elevations, elevations of streambed, and locations of cross sections for the Mendenhall River, Alaska: 7. Cross sections 100 to 92 A. For 2-, 10-, and 25-year floods. .24 B. For 50-, and 100-year floods . .25 8. Cross sections 91 to 74 A. For 2-, 10-, and 25-year floods. .26 B. For 50-, and 100-year floods . .27 9. Cross sections 73 to 57 A. For 2-, 10-, and 25-year floods. .28 B. For 50-, and 100-year floods . .29 10. Cross sections 56 to 48 A. For 2-, 10-, and 25-year floods. .30 B. For 50-, and 100-year floods . .31 11. Cross sections 47 to 42 A. For 2-, 10-, and 25-year floods. .32 B. For 50-, and 100-year floods . .33 TABLES 1. Summary of 2-, 10-, 25-, 50-, and 100-year discharges at the Mendenhall River and Montana Creek gaging stations, Juneau, Alaska. .15 2. Computed water-surface elevations for the 2-, 10-, 25-, 50-, and 100-year flood discharges, Mendenhall River, Alaska. .22 CONVERSION FACTORS AND VERTICAL DATUM Multiply by To obtain inch (in.) 25.4 millimeter foot (ft) 0.3048 meter mile (mi) 1.609 kilometer square mile (mi2) 2.590 square kilometer foot per year (ft/yr) 0.3048 meter per year cubic foot per second (ft3/s) 0.02832 cubic meter per second foot per mile (ft/mi) 0.1894 meter per kilometer Sea Level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929—A geodetic datum derived from a general adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level Datum of 1929. Elevations used in this report are referenced to Mean Lower Low Water (MLLW) which is a local datum. This datum is 8.2 feet below the National Geodetic Vertical Datum of 1929. Hydrology, Geomorphology, and Flood Profiles of the Mendenhall River, Juneau, Alaska By Edward G. Neal and Randy H. Host ABSTRACT INTRODUCTION Water-surface-profile elevations for the In 1997, the U.S. Geological Survey 2-, 20-, 25-, 50-, and 100-year floods were (USGS), in cooperation with the Alaska computed for the Mendenhall River near Department of Fish and Game and the City and Juneau, Alaska, using the U.S. Army Corps of Borough of Juneau, conducted an investigation Engineers Hydrologic Engineering Center to estimate water-surface profiles for flood River Analysis System model. The peak dis- flows of selected recurrence intervals on the charges for the selected recurrence intervals Mendenhall River in Juneau, Alaska. Increas- were determined using the standard log-Pear- ing development in the Mendenhall Valley cou- son type III method. Channel cross sections pled with the current rates of tectonic uplift were surveyed at 60 locations to define hydrau- have resulted in a need for refined flood pro- lic characteristics over a 5.5-mile reach of river files of the Mendenhall River. These profiles beginning at Mendenhall Lake outlet and will aid managers and planners in evaluating extending to the river mouth. A peak flow of and prescribing bank protection and stabiliza- 12,400 cubic feet per second occurred on the tion methods. Mendenhall River on October 20, 1998. This discharge is equivalent to about a 10-year flood The Alaska Department of Fish and on the Mendenhall River and floodmarks pro- Game and the City and Borough of Juneau are duced by this flood were surveyed and used to concerned with impacts of bank-hardening calibrate the model. The study area is currently projects on river geomorphology, the riparian experiencing land-surface uplift rates of about zone, and instream fish habitat on the Menden- 0.05 foot per year. This high rate of uplift has hall River. Currently, flood-plain managers use the potential to cause incision or downcutting water-surface profiles and flood maps from the of the river channel through lowering of the Federal Emergency Management Agency base level. Vertical datum used in the study (FEMA) (1990) flood insurance study. This area was established about 37 years before the study was based on surveys completed in the most recent surveys of river-channel geometry. late 1960’s. Established benchmark elevations The resulting difference between land-surface used in the FEMA report and for development elevations and sea level continues to increase. activities around the Mendenhall Valley were Continuing incision of the river channel com- last surveyed in the 1960’s. Hicks and Shofnos bined with increased land-surface elevations (1965) first documented ongoing tectonic uplift with respect to sea level may result in com- in southeast Alaska. Their results, later con- puted flood profiles that are higher than actual firmed by Hudson and others (1982), indicated existing conditions in the tidally influenced that rates of uplift in the Juneau area could be reach of the river. as much as 0.05 ft/yr. The National Oceanic Abstract 1 and Atmospheric Administration (1999) has (Barnwell and Boning, 1968) to more than determined that sea-level trends at the Juneau 12,000 in 1998 (K.J. Bailey, City and Borough tide gage have declined 0.04 ft/yr since 1936. of Juneau, written commun., 1999). The east These findings indicate that land surface may banks of the Mendenhall River have had sub- have uplifted as much as 1.8 ft in the study area stantial development since 1966, whereas (with respect to sea level) since the initial flood much of the west bank remains undeveloped. profiles were generated. Elevations in Mendenhall River Basin, an area of about 103 mi2, range from sea level to Purpose and Scope nearly 7,000 ft. A large part of the upper basin is covered by glaciers, whereas the valley floor The purpose of the study was to deter- is relatively flat and covered with streets and mine water-surface profiles for selected flood housing developments. Muskeg and spruce for- flows of the Mendenhall River. The City and est cover most of the undeveloped regions of Borough of Juneau and the Alaska Department the valley. The Mendenhall River flows out of of Fish and Game requested profiles for the 2-, the terminus of Mendenhall Glacier at the 10-, 25-, 50-, and 100-year floods. The dis- northern edge of the valley and into Menden- charges for floods of the selected recurrence hall Lake. Mendenhall Lake receives additional intervals were computed using standard log- inflow from Nugget Creek, Steep Creek, and a Pearson type III frequency analysis. The water- few small tributaries. After exiting the lake, the surface profiles that would result from these river flows through the Mendenhall Valley in a floods were determined for a 5.5-mile reach of generally southward direction until it enters salt the river using the U.S. Army Corps of Engi- water in Fritz Cove. neers (USACE) Hydrologic Engineering Cen- The Mendenhall River provides aquatic ter River Analysis System (HEC-RAS) model habitat for chum, pink, sockeye, and coho (U.S. Army Corps of Engineers, 1997a, b, c). salmon; cutthroat and steelhead/rainbow trout; This report also includes discussion of the and Dolly Varden. The river is also used as a Mendenhall River geomorphology and relates migration route for fish bound for Montana and river morphology and flooding potential to Steep Creeks and several small unnamed regional land-surface uplift, which is ongoing creeks flowing into the west side of Menden- in the study area.