Debris will continue to accumulate below the waterfall unless the villagers keep it clear. This is one way to make sure that there is something for builders to use to develop their skills, and also to control the flow of the stream. If the stream is not kept clear by the villagers, the percentage of obstruction will gradually increase until the stream is once again blocked, changing the flow of the water.
Unlike the projects for structures, the percentage displayed for the stream project represents the amount of blockage. When the stream is completely blocked, the obstruction is 100%. As the villagers clear the obstruction, the percentage decreases until the stream is completely cleared, and the percentage of blockage is zero.