They don't/didn't need to. The dam itself had caused such a massive silt build up that the overall outflow after the breach was virtually no different than before. Essentially the silt had back up on the reservoir side of the dam to the point that it nearly reached the top of the dam. That means there wasn't a big wall of water coming down after the breach, all it did was shift the flow from over the top of the dam to the breach itself which allowed the water to carry that built up silt and sediment on downstream where it will eventually wash out into the bay as it should have been doing these past 100 years or so. This is returning the river to its proper flow and run for the first time in nearly a century.
How did they stop the water??
They don't/didn't need to. The dam itself had caused such a massive silt build up that the overall outflow after the breach was virtually no different than before. Essentially the silt had back up on the reservoir side of the dam to the point that it nearly reached the top of the dam. That means there wasn't a big wall of water coming down after the breach, all it did was shift the flow from over the top of the dam to the breach itself which allowed the water to carry that built up silt and sediment on downstream where it will eventually wash out into the bay as it should have been doing these past 100 years or so. This is returning the river to its proper flow and run for the first time in nearly a century.