The Mill Avenue Bridges consist of two bridges that cross the Salt River in Tempe, Arizona. The first bridge opened in August 1931 but was not dedicated until 1 May 1933. The dedication celebration lasted for two days. Attending the celebration was Benjamin Baker Moeur, a former Tempe doctor and the governor at that time. The creation of this bridge replaced the Ash Avenue Bridge which was a dilapidated, one-lane, wagon bridge just to the west. It was demolished in 1991. In the Phoenix area, it was the sole crossing at the Salt River for some time.
The New Mill Avenue Bridge (located directly to the east) opened in 1994 to relieve the original bridge from the increasing traffic. This allowed for two lanes to travel in each direction (North and South), instead of the previous single, two-lane bridge.
Water flowed down the Salt River until the 1940s, when dams were constructed upstream. The water flow practically stopped, creating a dry river bed to support the growing Southwest. For years, southbound traffic used both lanes of the bridge, while northbound traffic utilized an unbridged crossing in the riverbed.
Despite the Salt's being a dry river, water occasionally flowed. When reservoir levels got too high, the dams were required to release water, causing water to flow once more. Due to monsoon storms heavy rains would fall, and washes and street runoff emptied into the river. At such times the unbridged crossing was closed, and the bridge was opened to north- and southbound traffic, one lane in each direction.
The bridge faced many strong floods that raged through the Salt River Valley. In 1980 all but two of the bridged crossings on the Salt River were closed for safety reasons (principally erosion of the approaches) due to severe flooding. The Mill Avenue Bridge and one other bridge in Phoenix were the only bridges that remained open. This was because they were structurally sound to stand up to the raging currents. Water hit the bridge at 200,000 cubic feet (5663.37 cubic metres) per second, which far surpassed the expected strength of the bridge. In one 24-hour period during this flood, 92,000 vehicles crossed the bridge.
With the opening of the northbound New Mill Avenue Bridge in 1994, the unbridged crossing was permanently closed. with two lanes now running in each direction no matter the weather, monsoon storms and releases from dams no longer lead to traffic obstructions.
In 1999 the dry river bed was transformed into a dammed artificial lake. Tempe Town Lake was a key success to the revitalization of Downtown Tempe. Before, just a crossing over the dry river, these bridges became a centerpiece of the new lake. This prompted a lot of development along the lake. Today, mid-rise offices rise above the southeast portion of the bridges.