Tuolumne River, Installation of a Permanent River Gage above Hetch Hetchy Reservoir

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This project involves the installation of a U.S. Geological Survey river gage on the Tuolumne River approximately 300 feet upstream of Hetch Hetchy Reservoir. This gage will record and transmit in real time the flow rate, temperature,turbidity, and conductivity. The project is funded through the San Francisco Public Utilities Commission.

The actual installation would consist of a weather proof instrument box (approximately 2 feet high by 3 feet wide by 2 feet deep) mounted to a 3-4 foot high metal stand that would be bolted to bedrock using four ½ inch diameter holes. The exact location of the gage house would be determined at the time of installation based on the elevation of high-water marks of prior events and terrain suitability. A 1.5-2 inch PVC line or a 1 inch galvanized pipe will extend from the gage house approximately 100 feet to the water's edge. The conduit would contain cabling for the depth, temperature, conductivity, and turbidity sensors and will be secured using 4-6 tie-downs bolted to the rock. The line would be concealed as much as possible without trenching by following the contour of the land surface and/or open joints in the rock. A series of three staff plates (ruled vertical signs necessary for the measurement of river level) would be tiered from the water's edge up to the gage house. Plates would be mounted in the rock using two ¾ inch holes backfilled with pour-rock cement. Crest-stage gages, which consists of 4 foot long 2-inch galvanized vertical pipe with a stick running the length of the pipe would be installed on the two upper staff plates to verify peak flows as measured by instrumentation. A total of up to 20 holes would be drilled into bedrock. A solar panel (approximately 36 inches by 24 inches) would be installed to power monitoring and satellite telemetry equipment. It would be mounted on a pole attached to the gage housing and be positioned up to 5 feet above the structure as necessary to obtain a clear view of the southern sky. A satellite antenna (approximately 48 inches in length by 10 inches in diameter) would be installed to transmit data on a near real time basis.

Projected crew during construction would be four people over 3-4 days. Access would be by boat provided by Hetch Hetchy personnel. An effort would be made to construct or fabricate as much of the shelter and hardware prior to installation, thereby lessening the need for intensive use of heavy equipment. Motorized equipment required for installation includes a generator and a hammer drill.