One might be surprised to hear that even though the rivers rose, some areas were less affected by the flooding. In the case of my home county, Stanislaus, there were (and continue to be) problems along the lower reaches of the Tuolumne and San Joaquin Rivers, but on the east side of the valley there were few ill effects. Along my usual walkway, the Tuolumne River Parkway Trail in Waterford, the damage was of a type not often associated with a flat valley floor: mass wasting, or mass movement.
Mass wasting happens because of gravity, but an overaccumulation of water can substantially add to the intensity and degree of movement. The movement takes three forms: falls, flows, and slides. I saw examples of all three this week.
In the picture above, there was so much water built up in the soil that the slope failed rapidly and the fluid mix of silt and water flowed and covered part of the trail below. This is called a mudflow. In different circumstances, especially involving glaciers and erupting volcanoes or desert cloudbursts, mudflows are one of the most dangerous forms of mass wasting. A single volcanic mudflow in Colombia in 1985 killed some 25,000 people.