It is no secret that I am not a big fan of this project so take what you will from my posts. Questions answered, Yes and no. They have lowered the dam to help with the restoration but will replace it but not at the same height it is now. Supposedly it will only be "a couple inches lower". They are removing "evasive and native plants ( privets and honeysuckle) and replacing them with dogwoods and viburnums. That should make for a nice garden setting!!!!!
It sounds to me like they are really on the right track. Removing invasive shrubs and replacing them with native shrubs is a good thing. And riparian shrubs and trees grow rapidly.
And keeping the dam but lowering the elevation a bit is exactly the right solution, IMHO. I can't think of a short or easy way to explain why, but here goes:
The Yellow Breeches, like most streams in PA, is a highly altered stream. It's nothing like it would have been in 1600. It's simplified, straightened, disconnected from its floodplain, there's development in the floodplain.
And this is why the Breeches, like many of our streams, is lacking in good pools and cover. Pools often form at meander bends. When streams are straightened, they lose meander bends pools. Streams also have much less large woody debris than in the past and LWD is also a big cause of pool and cover formation in streams. And streams originally had channels that split, then rejoined, and confluence pools were created at the re-joinings.
The simplification (channelization) of streams greatly reduces the processes that form pools and cover. The result is a lot of flat shallow featureless water.
In simplified streams, often much of the pool habitat that does exist is created by artificial structures such as dams and bridge abutments and rip-rap and even sewer line crossings.
It's artificial structure, but it's structure, and that beats no structure.
That dam creates a pool above the dam, and nice runs below the dam.
The height of the dam, though, is higher than optimal. It backs the water too far upstream, i.e. the pool is too long. This causes the water to slow too much in the pool, and for large amounts of sediment to drop out, making most of the pool shallow.
Many people think that a pool behind a dam inevitably will fill in with sediment. But that is not true. It does that if the pool is backed up too far, so that the water slows too much.
If the pool backed up behind a dam is the optimal size, high velocity flows coming into the pool from above during high water events will keep the sediment flushed out and the pool will remain deep. I've seen this many places.
So, if they lower the elevation of the dam a bit, it will reduce the surface area of the pool. It won't extend as far upstream or be as wide as before. But it should result in a deeper pool, with better current flow into the head of it, so result in a higher quality pool that will support more trout.
