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COMPLETED PROJECT

Project Report:

Open-File Report 2008-1098
Bathymetric Survey and Storage Capacity of Upper Lake Mary near Flagstaff, Arizona
by Nancy J. Hornewer and Marilyn E. Flynn

Storage Capacity and Areas of Leakage for Upper Lake Mary, near Flagstaff, Arizona

Project Number: 9671CJF
Project Chief: Nancy Hornewer
Cooperating Agencies: City of Flagstaff, Arizona
Period of Project: July 1, 2006 – September 30, 2007

View of Upper Lake Mary

PROBLEM

Upper Lake Mary Reservoir serves as a source of drinking water for the City of Flagstaff. Therefore the lake’s storage capacity and sedimentation issues are of interest to the City of Flagstaff. The City also is interested in locating fracture zones beneath the lake where seepage losses might occur.

OBJECTIVES

The first objective for this study is to determine the storage capacity of Upper Lake Mary. The second objective is to determine the feasibility of locating and characterizing fractures beneath Upper Lake Mary using ground-penetrating radar (GPR) with sediment concentrations to calculate sediment loads.

APPROACH

To determine the storage capacity of Upper Lake Mary, elevation measurements must be made from the lakebed to the elevation of the spillway. A bathymetric survey, using a single-beam echosounder, will be completed in late summer to determine lakebed elevations. Water-depth measurements will be made at approximately 40-meter (m) intervals throughout the lake where water depths are greater than 0.5 meter. Elevation measurements from the lake surface to the elevation of the spillway will be made using land-surveying equipment. Horizontal-position data will be collected using a Global Positioning System (GPS).

To determine the feasibility of locating and characterizing fractures beneath Upper Lake Mary using ground-penetrating radar (GPR) equipment a short test will be conducted in late winter or early spring. Various antenna-frequency configurations will be tested on the GPR unit (Sensors and Software Pulse EKKO 100) to optimize measurement resolution for fractures and sediment depths. Data will be processed and evaluated to determine if GPR is suitable for locating and characterizing fractures in this environment. GPS will be used to collect horizontal-data position.

RELEVANCE AND BENEFITS

Results of the proposed work will provide the City of Flagstaff with updated information on the bathymetry and storage capacity of Upper Lake Mary. GPR and shallow-seismic feasibility study results will be evaluated to determine whether further investigations could identify highly-fractured areas within the lake. If results look promising, further work could include a comprehensive evaluation of the lake bottom for fractures coupled with seepage measurements to locate areas where seepage losses might occur. If specific areas can be identified, the City might be able to fill the fractures reducing seepage losses from this important source of drinking water.

This study meets several USGS objectives including advancing the knowledge of a regional hydrologic system and advancing field or analytical methods. If the techniques used in this study are successful, the same approach can be used in other applications regionally and nationally. If successful, these methods provide another tool for evaluating recharge potential from and water availability to lakes and riparian habitat. This information would be of great value to managers because water is in such shortage.