As water flows into Panther Hollow Lake from Panther Hollow Run, it slows down. This causes it to drop sand and other coarse sediment near where it enters the lake. The resulting formation is a small delta, with features also found in much larger deltas.
This delta, as well as silt and clay deposited elsewhere in the lake, are leading to the lake filling with sediment, which results in a smaller, muddier, and more polluted (due to shallow water becoming warmer in the summer) lake.
Although it is not readily evident, largely because changes in water level affect how large it looks, the delta is slowly expanding.
The following is a progression of photos of the delta, from the Schenley Park Bridge. Changes in the size of the delta are in part due to the water level in the lake, but you can still get a feel for how the delta changed over the last 6 months or so.
June 4, 2010
June 7, 2010, after flash flooding. Note the much higher lake level.
July 19, 2010
July 26, 2010
Jan 10, 2010.