Hydrology – 26 SEEPAGE LOSS

Hydrology of Crater, East and Davis Lakes, Oregon by Kenneth N. Phillips




The fact that East Lake water is fresh indicates that it must lose water by seepage in an amount not less than the flow of the mineralized springs that enter it. Fragmentary records of the stage of East Lake provide a clue to the general order of the rate of seepage loss. If all the precipitation in its basin reached the lake, except for the annual evapotranspiration loss of about 12 inches on the tributary area of 3,830 acres, then the estimated annual evaporation of 28 inches from the lake plus seepage loss of 10.9 cfs would be needed for balance. The seepage rate may vary with lake level, but, for a given level, it should be the same in a wet year or in ‘a dry year. However, in a very dry year, when the total precipitation at East Lake is only about 21 inches, the evaporation would exceed the precipitation, the runoff to the lake could be no more than about 2,900 acre-feet, and leakage at a rate of 10.9 cfs would then lower the lake level at a calculated rate of 6.5 feet in 1 year. No such rapid decrease has been observed; in fact, in the year ending September 30, 1959, when the precipitation at Bend was only 53 percent of the 1931-60 normal and that at East Lake was probably not more than 21 inches, the water level dropped only 1.7 feet. The lake level was high in 1959, and the rate of seepage therefore must have been at least as much as the average long-term rate. It seems obvious that the seepage is much less than 10.9 cfs.

The assumptions and computations shown in table 9 lead to an estimate of about 2.3 cfs seepage out of East Lake. Such a rate is much more than the combined flow of the observable mineralized springs. The estimated average annual inflow to East Lake of 1,700 acrefeet (table 9) is only 23 percent of the estimated difference (7,340 acre-ft) between the precipitation of 35 inches and evapotranspiration of 12 inches on the basin area (3,830 acres.) Therefore, the tentative, conclusion must be that a large part of the basin topographically within the East Lake part of the caldera does not contribute any water to East Lake but instead drains’ water away from it.

The hypothesis suggested above, that East Lake basin is not all tributary to the lake, is based on sketchy hydrologic information and should receive further study when the required data have been obtained. Continuous records of lake level and precipitation are needed.

Data are not adequate to define the relation of seepage rate to lake level. It is not even certain that seepage occurs when East Lake is below the minimum known level of about 6,366 feet reached ‘about 1941. It is probable that seepage is most rapid when the water level is high.

There ‘is no hydrologic information to suggest where the seepage outflow reappears.

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