I don't think there is any worry on the heavy water influx. The runoff is all from the same, original water shed, so there should not be any pH or other water quality differences....Further, the lowered lake will re-fill over hours, not minutes, so the fish will easily temper as the fresh water enters. In fact, due to the concentrated bio-mass of fish, I would bet the influx reduced some rising ammonia and nitrite levels...the fish most likely benefited with the fresh water.

Also, even if there is a big temp difference from the lake and runoff, the waters will not mix instantly, leaving many safe pockets for temperature acclimation (15-30 minutes) to occur.

My biggest concern is what esshup mentioned about the concentration of fish. If this is a mature lake and lowered, depending on what is being related as "about 50%", the lowering itself could put the fish at a big risk, from predation and rapidly deteriorating water quality.

If lowered 6' (50% of deepest point), depending on bottom slope there could be 80+% of the lake's total water volume removed.