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DaveQuote
Frank Furter
No comments about Mirror Lake? Why not resume dredging and perimeter vegetation removal to restore its prior glory?
What's happening in Mirror Lake is a natural progression. What's happening in the meadows, and other places, are the result of human interference.
I did a little research today about the formation of Mirror Lake. It appears that the present Mirror Lake wasn't formed by a glacier moraine dam, but instead by a dam created by some giant rockfalls from the surrounding cliffs.
Here's an excerpt of a USGS publication in regards to the formation of Mirror Lake:
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USGS: POSTGLACIAL HISTORY OF THE YOSEMITE VALLEY
FILLING OF LAKES IN TENAYA CANYON
Although the steps in the floor of Tenaya Canyon are not especially clean-cut, their configuration nevertheless indicates or at least suggests the former existence of a glacially scooped lake basin on each tread. There were apparently four such lake basins in the stretch extending from the mouth of the canyon to the great step from which the Tenaya Cascade descends. The upper three were little more than ponds that must have been quickly filled, but the lowermost, near the mouth of the canyon, measured over a mile in length and probably was not filled until a long time after the glacier had withdrawn from the canyon.
Mirror Lake (pl. 47, A) might readily be supposed to be a remnant of this ancient lake—in fact, it is generally assumed to be of glacial origin, like most lakes in the High Sierra—but a scrutinizing study of it reveals that it is of relatively recent making and has had no connection whatever with the ice age. It is impounded wholly by masses of rock débris that fell in avalanches from both walls of the canyon, principally from a place on the west wall, just back of the Washington Column. West of the outlet of the lake, it is true, there is a small quantity of morainal débris, readily distinguishable from the angular avalanche material by the water—rounded and glacier—polished cobbles and pebbles which it contains; but there is no dam composed of such glacial material across the canyon.
The creation of Mirror Lake by obstructing rock avalanches was of course favored by the fact that the lower part of Tenaya Canyon already had a nearly level floor due to the previous filing of a glacial lake basin. Had the floor not been so nearly level, the low, irregular avalanche dam would not have backed up the water for any great distance.
The postglacial history of lower Tenaya Canyon may be summed up as follows: (1) A glacially gouged lake basin about a mile in length was filled by the forward growth of a delta built by Tenaya Creek; (2) great rock avalanches fell from the walls near the canyon mouth and impounded the water anew, forming Mirror Lake; (3) Tenaya Creek built a new delta, which, growing forward, is now reducing the size of Mirror Lake.
The relative recency of its creation therefore explains why little Mirror Lake is still in existence, whereas the large lake basin in the Yosemite Valley is completely filled. Mirror Lake itself, however, appears to be not far removed from extinction. The rate at which the delta at its head is encroaching upon it from year to year is sufficiently rapid to be discernible to the eye without the aid of precise measurements. Indeed, unless steps are taken to check the further growth of the delta, Mirror Lake will soon be reduced to a small, unimpressive pond, and the Yosemite region will lose one of its most valued scenic treasures.
So in other words, while the present day Mirror Lake is slowing dissolving into a meadow, a couple of more giant rockfalls at the right locations could easily create an even larger Mirror Lake in a not too distant future. So dredging might not be necessary after all
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