When the gound cannot absorb the amount of water that is coming down in heavy rain, or released by the melt of the snow pack, then flooding can occur. It can be a very dangerous situation. As rivers and creeks are unable to contain the extra participation, banks overflow into the surrounding areas filled with people. Homes and vehicles can be washed away, power outages and mudslides can occur, people can be left stranded. Here is a link to a video from Alaska State Troopers, titled Flooding in Alaska.
Another link to a video that shows how an avalanche cuts off Valdez. Water is pooling behind the avalanche and could potentially cause flash flooding: https://usnews.nbcnews.com/_news/2014/01/28/22481494-officials-monitor-lake-created-after-avalanche-cuts-off-alaska-town