About Those Cableways
United States Department of the Interior
Mr. Shane Murphy
Dear Mr. Murphy:
Thank you for your letter expressing the concerns of the river guides and your offer of assistance to remove the USGS cableways in the Grand Canyon. We understand and share your concerns about the visual impact of cables in the Canyon. We have examined other measurement alternatives to the cableways, but unfortunately, another method does not exist at the present time to measure discharge and collect water samples at flood stage with acceptable accuracy.
The cableways above the Little Colorado River and at National Canyon are planned for removal after the experimental high release that is still scheduled for April 1995 [see page 2]. When these cableways are removed, it will be because the gages are being removed, not because we have an alternative method of measurement.
The gage sites, cableways included, have always been considered temporary, although changing goals and direction of the GCES program have resulted in these sites remaining in operation for more than the 2 years, as originally proposed. Although it is true that GCES was originally conceived as a 2-year project, it was known from the beginning that 2 years of data were not sufficient to develop predictive models of flow and sediment transport. This is particularly true because the initial purpose of the project was to investigate the effects of unsteady powerplant releases on sand deposits, when only 3.5 months of such flows had occurred in the first 4 years of the project. Our knowledge of flow and sediment transport in the Canyon has improved significantly since 1983, as has our ability to predict response of sand deposits to tributary inflows and dam releases. We are, however, just getting to the point where predictions are accurate enough to have practical management application. Our flow and sediment-transport models will continue to improve as more data are obtained, more is learned, and more testing is done. As the models improve, the need for direct measurement will decrease, and it is one of the goals of our program to reduce that need. However, some direct measurement will always be required if the Canyon is to be managed in an environmentally-sensitive way. Some gaging stations will be a part of the long-term monitoring program. The number, location, and data collected at these sites has not yet been determined.
Our cableways at the sites and others have a two-fold purpose. One is to allow our technicians to measure the discharge of the River. Measurements of discharge are used with stage recorded by sensors at the gaging station to develop the relation between stage and discharge used to compute discharge for all record stages. The other function of a cableway is to make possible the collection of sediment samples that correctly characterize transport across the entire River.
Cableways have several advantages over other methods: (1) they do not physically interfere with boat traffic; (2) they do not interfere with the flow of the River and so provide the best means of collecting samples and measurements that are unaffected by surface interference; (3) they do not require boat operation once the crew is at the site; (4) they are already in place and so less cost, time, and effort are required to make measurements and collect samples. We believe the cables offer the safest, lowest cost, most feasible means of collecting data of the required accuracy during the high-flow experiment.
If we maintain gaging stations at any of these sites, then we have to be certain that changing methods of data collection don’t make the data incompatible with previously collected data. There has been a 50-year squabble about the effect of changing from the Colorado River sampler to the modem suspended-sediment samplers. Most people now believe that the decrease in suspended-sediment loads in the Colorado River in the 1940s was real and not the result of changing samplers. The question would have been easily solved, however, if both samplers had been used for a long enough period of time that the results could have been compared. Because the Survey has a long-term commitment to the Nation, we have the responsibility to ensure that measurements and samples taken now will be compatible with those collected 50-100 years ago and those to be collected in the future. This commitment may mean that we are slow in adopting new technology, but it ensures that data collected today will be useful in the future.
The removal of the cableways will be costly—an estimate of $20,000 to $25,000. When we do remove the cableways, help from the outfitters and guides would reduce our costs, and would be greatly appreciated.