Oooh, don't get me started talking about BDA's!!
To the fish question - they actually don't hamper migration. As weird as that sounds to say, as contrary as you'd think that is, they actually don't bother the fish at all. In the Western US, there's a saying that beavers taught the salmon how to jump.
In the UK, they've done quite a few studies on reintroduced beavers and found that fish populations are actually higher after the beavers build their dams.
The dams create a plethora of currents downstream because the water leaks out slowly an in different spots. They aren't solid masses, so fish can pick and choose which current best suits them as they wiggle their way through the things.
The BDA's mimic the real beaver constructions as best we can. Even little things like having the branches sticking out on the downstream side was found to be very important because it prevented scouring at the base of the dams, and the subsequent blowouts, while also forcing the water to tumble - thus aerating it and improving the balance of nitrogen, etc. Very happy fish, birds, and bugs are the result!
As you note, the woody structures slow the pace of the water. This is great for giving it time to seep into the ground, restocking the water table. It also creates cleaner, clearer water because slowing the flow down allows sediment to fall out. Thus, you stop pollution and erosion.
There are many cases where a few BDA's along an incised stream were able to slow the flow enough that they raised the stream bed 3' per year until it finally reunited with its flood plain.
Contrary to popular belief, BDA's don't have to cross the entire stream. Like natural beaver dams that blow out during high rain events, you can use partial BDA's to force the water to the side, increasing sinuosity in the flow of the river and thus slowing it down. Creating meanders in the stream makes it prettier, increases habitat variation, and betters the fish, birds and bug populations.
By spreading out the flow of water, you can help restock the water table, enrich the soils of the flood plain, kill non-native species that often encroach onto territory that's now too dry for native plants, and increase the overall health of the ecosystem.
One of the important things to remember, and a problem we share here in the States, is that your entire ecosystem evolved with the Beavers. When the beavers were eradicated in the UK, it caused a cascading collapse because beavers are what is known as a Keystone species. The habitat they build is crucial for everything else in the food chain.
It's really weird to try wrapping your mind around, I know. Even though you've been to place you think are pristine wilderness, they are really not. Nobody alive today has ever seen Britannia as it's supposed to be.
Here's a picture of one of the first beaver dams to be built in England in over 400 years. Pretty cool, huh?
Here's a nice little dam and pond in the Devon Valley where beavers have gotten stuck in and are being watched.
This river bottom was once just a channel that drained the water out of the whole region just as fast as gravity could make it happen. It wasn't supposed to be that way, but the beavers haven't been there for 400 years.
As soon as they were returned, they got to work and the actual natural state of England responded accordingly. This is what the land is supposed to look like. This is what it looked like for millennia.
Beaver Dam Analogues aren't nearly as good as real beaver buildings, but they go a long way towards bettering the environment both upstream and downstream of where we stick them in.
The downside is that BDA's require work on our part. We build them and we maintain them. We've got to get our hands dirty rather than leaving it up to the beavers.
The upsides, though, far outweigh the downsides. Even when a BDA gets broken by a storm, the remains still serve a valuable purpose in the local ecology and can help to both shape the waterway and clean it.
Of course, that also means you have to get back in there to either repair it or install a newer, better one.
For a small-time land owner or custodian, the principals of the BDA can be used to mitigate runoff and the development of gullies. Slowing the water down with even something as simple as a swale made from downed tree sections will trap sediment on the uphill side and slow runoff so it had time to sink into the ground.
If you have a little stream running through your property, installing a few BDA's along its length will pay huge dividends in just one season as they raise the bed, aerate the water, and create variations in the current strengths that are better suited for various fish and bugs.
Here's short BBC documentary on the beavers that were returned to one small study area....
Happy British Beavers Beavering Away!!!
As for fish getting through... the beavers are happy to build little sluices for them....
It's not either or....
Regarding the paddlers.... well, phooey on them, I say.
I like canoeing as much as the next guy... and I hate portaging. That said, we can't get beavers or BDA's everywhere we need them. There can be some give and take. And, really, the best place for BDA's isn't in the big waters that people like to paddle around it. By then, the water usually has too much power, especially in the rainy season.
Instead, we should be focusing our efforts up in the tributaries, the small streams and creeks that are just trickles. These are the guys that suck the water out of the highlands, and it's exactly where we need to concentrate our efforts if we want to see the benefits of slower, cleaner water hitting the lowlands and valley bottoms.
It's really easy to build a handful of BDA's along a trickle you can step across. And because these tiny runs don't get the huge volumes of runoff seen in the bigger waters, a few small BDA's can be built in a weekend and make a tremendous difference.
Plus, it's just fun to build dams. If beavers can do it without chainsaws and heavy equipment, any human should be able to as well. It's just a matter of strapping on the boots and getting at it.
Great project to get the kids involved with, too.
