Their confluence occurs just downstream of the upper Santa Ana Canyon mouth.

Mill Creek is in much better condition than the lower portions of the Santa Ana watershed, as its drainage area is less urbanized.

The first commercial power plant in the United States using three-phase alternating current was the 250 kilowatt Mill Creek No.

Also, a levee system was built by the United States Army Corps of Engineers after a severe flood, which degraded the riparian habitat.

In addition, the poor conditions downstream interfere with fish migration to the creek.