Until I crossed the gangway over the dam enroute to the Hall Natural Area in the Mt Conness drainage, I wondered why Saddlebag Lake had been drawn down to little more than a creek below the base of the dam.
The answer appeared when I crossed: a gaping hole about a meter in diameter. As I understand it, with enough pressure (e.g., full head in springtime) even a much smaller hole could drill a 'pipeline' right under the dam, and possibly cavitate and ultimately undermine the bedrock upon which the dam rests.
Judging by the size of the hole it looks to render the dam inoperable until repaired. I did not investigate below the dam on this day, since I was on a schedule, and wherever that water has been going it could be emerging far below. While the size of the entry hole might be much larger than where water has to squeeze through, when the reservoir is at capacity as in 2017, there must be ~70 feet of head above the hole (more as it deepens)—that’s a lot of pressure against the mostly metamorphic rock underlying the dam. Mix in some gritty sand for scouring and that relatively soft rock might wear away quickly.
It’s ironic that all the new cladding installed on the face of the dam a few years ago is for naught until this breach is fixed. I’m no dam engineer but it doesn’t seem so simple to me to fill in a hole that burrows under a dam. Seems to me that one cannot just pour a concrete slurry down the hold and hope it holds, but that the entire frontal area would need to be scraped to bedrock, and a thick cap of concrete poured over it all.
Meanwhile, most of the trophy trout that I saw swimming around in July in this very place are surely still holed up in the much-reduced lake volume—gotta go fishing this fall! Fish and game regularly releases some monster (as large as 28 inches) into area lakes.