Flooding in Tucson, Tucson
Tucsonans and other desert-dwellers are often made fun of for the idea that we freak out about a little rain. What people fail to understand though is that Tucson really does not have any sort of drainage system: it's all natural. Streets gradually flow into washes, which gradually flow into one of our "rivers" and then the water sort of gets lost in the desert near Casa Grande. It's the first and second parts of that that tend to cause problems.
There are many streets that flood often and quickly. Too many to list really, and you just have to know. If you're new in town, you can ask locals, but radio stations will typically announce which streets are flooded and/or impassable when it rains heavily. Pima County also maintains an updated online database of road closures, in both list and map format: http://dot.pima.gov/gis/maps/closures/
This also makes walking around very difficult. It's a good idea to get waterproof boots if you plan on relying on buses or walking, especially during the summer, because you will likely be forced to cross flooded streets at some point.
The Santa Cruz and Rillito Rivers typically don't cause too many problems when they fill with water. There are a couple of low-water crossings of the Rillito, but these can easily be avoided. There are several washes that cause problems in the suburbs: Pantano, Tanque Verde, and, particularly, Canada del Oro. The latter is so problematic because of the amount of development in that region and the lack of proper infrastructure. These washes have many grade crossings, where the road turns to concrete and actually crosses the wash at a slightly raised level, as opposed to going over it on a bridge. When these washes flood, the roads are impassable. It sometimes takes time for the sheriffs to close off a road, so going through is at your discretion, but it's just best not to try it if you think it's high. There are often "Do not enter when flooded" signs at these crossings to remove the county from any liability, but they should be taken with a grain of salt. For instance, a flowing wash over a road with only a couple of inches of water may freak out many, but it's easily passable. It's a foot or more that causes problems.
Arizona also has what is referred to as the "Stupid Motorist Law" where, if a street has been closed due to flooding, but you take the risk anyway and have to be rescued from your vehicle, you are charged for the cost of that rescue. You may think "Who would do that?" but it seems there are at least 20 of these incidents each year in Tucson alone. Not only that but, if you're with children, you could be charged with child endangerment.
Heavy rains during the monsoon season can cause sudden flooding on roads that cross washes. Use caution when crossing dry washes in the rain, and never attempt to cross a road that has been flooded. Floods occur often on streets near the University of Arizona's main campus, and under the bridge near Toole.
It's not supposed to rain in Tucson...they are surrounded by mountains which cast a prepetual rain shadow! Not during my stay! It rained two days straight in Arizona from the 22-23 of December. A little rain goes a long way here in the desert....Depending on what time of year you go, make sure you are aware of the dangers of flooding. I was driving around and there are thousands of signs that say "Do not enter when flooded," and markers that went up to 8ft!!!!! They are not kidding when they talk about lack of drainage!
Monsoon Season. Late July and early August is the rainy season. Since there are no storm sewers, the water fills the streets. Flash floods are common.