The first day without people Hoover Dam could continues to work for couple of years. After 10 day, The Rats and Mice try to survive on once they have gone through human produced food are the food that human have left over. 6 months later, the 1st to colonize our vacated urban areas are small perdators. A year later, the Colorada River once the dam shuts down will dry. 5 years, the reason why these animals be perfect for survival because at that time the city is just look like a forest. 20 years have pass, all the animals at that time will make their new house in the city. 25 years, in Londo and Amsterdam will look like a pool. 40 years, the big enemy of stone structures are salt will grow and break in a pieces. 50 years, the thing that happens to the bridges of the world at this point is they will corrode. 75 years, the thing that left of cars are frames and skeletons.