The Sun Valley Watershed has been the site of human activity since at least 2000 B.C., when villages were typically established near permanent water sources, including the Los Angeles River. The Sun Valley area later became a portion of the lands allotted to Mission San Fernando, and ultimately became a part of the United States as a result of the Treaty of Guadalupe-Hidalgo in 1848. The land, then known as “Rancho Tujunga”, was divided and sold many times, and was quickly developing during the 1870s. By the year 1891, Sun Valley was well known for its vineyards and orchards, row crops, and wheat.
The Sun Valley Watershed is covered with rock and gravel deposits originating from the San Gabriel Mountains. This rock material was transported by storm runoff, and thus is a testament to the history of flooding in the area. In the early 1900s, large quantities of stone from Sun Valley were sold to be used for the Los Angeles Harbor and railroad lines were extended to facilitate the transport of rock and gravel material out of the region. The City of Los Angeles annexed the Sun Valley area in the 1930s and development and populations continued to increase until present-day.
The decades of urban development in Sun Valley have resulted in about two thirds of the ground being covered by hard, or impervious, materials. Surfaces such as asphalt and cement do not allow rainwater to soak into the soil, so it flows over the pavement instead. Since Sun Valley is a relatively flat area, stormwater travels over streets slowly in comparison to other hilly regions. In many parts of Los Angeles, storm drains help carry water away, but there are currently no major drains in Sun Valley Watershed. Even a moderate rainfall quickly overwhelms the few minor drains, and rainwater backs up on streets and in low-lying areas.