The branches of the fallen tree are catching debris and creating a dam across the creek. The dam slows runoff water and creates an upstream pool.
Moving water bounces small bits of gravel along the streambed. As the water reaches the dam, the velocity slows and loses the energy necessary to move the gravel. The gravel begins to pile up against the upstream side of the dam. Gravel will soon fill the pool and begin to fill areas farther upstream.
The water flowing over the dam creates a downstream plunge pool. The falling water has enough energy to keep anything from settling here.