For example, this is a bridge across the Dadu River under normal conditions:



More details here as well, with some images of the low flow downstream. It appears that the authorities are organising a large-scale evacuation.
The landslide has formed a 250m-long barrier blocking the local Dadu River. By 3:30 a.m. Friday, the water had overrun the obstruction, posing a danger to the lower reaches of the river. The runoff at the power station's dam is 240 cubic meters per second, compared with a flow of 2,760 cubic meters per second Thursday. The provincial government has ordered an evacuation of the people living at the lower reaches of the river to avoid further casualties. A provincial-level working team is also rushing to the scene to direct rescue work."
The Dadu River is famous for the disastrous valley-blocking landslides after the 1786 earthquake, which are believed to have killed 100,000 people when they failed. This area was also affected by the shaking from the Wenchuan Earthquake.
The report is somewhat contradictory about whether the river is still blocked, but at this time of year, and considering the amount of rain that the area has received this monsoon, the flow rate into a lake will be high.