Immediately to the east of the Arroyo Seco Watershed is the Rio Hondo Watershed. Like the Arroyo Seco, the Rio Hondo and its subwatersheds feed the Raymond Basin Aquifer and are part of the Los Angeles River system. Theres are two sister watersheds uniquely linked in geography and function.
Here is the description of the Rio Hondo Watershed from the Rio Hondo Watershed Management Plan.
"The Rio Hondo watershed is a subwatershed of the Los Angeles River watershed and is also linked to the adjacent San Gabriel River watershed. This link reflects both natural hydrologic processes and human intervention. Historically, the Los Angeles and San Gabriel Rivers were wide shallow rivers consisting of a braided series of channels that would periodically intermingle following large storm events. As a result of this dynamic, the Rio Hondo once formed the main bed of the San Gabriel River. Today, however, this dynamic has been engineered into three channels have been created to bring water from the San Gabriel to the Rio Hondo, making the Rio Hondo serves as a distributary for the San Gabriel River.
"The Rio Hondo watershed is also unique in that two very different landscapes shape the overall character of the 142 square miles that fall within the boundaries of the watershed. The rugged steep terrain of the San Gabriel Mountains defines the upper reaches of the watershed, much of which lies within the Angeles National Forest. This natural undeveloped landscape changes dramatically below the foothills where the nature of the watershed is transformed by the very urban and largely built out San Gabriel Valley. Encompassing 21 different cities and unincorporated portions of Los Angeles County, this densely developed urban landscape includes only a few remaining areas of open space and isolated patches of natural habitat."