The Rio Hondo (In Spanish "Deep River") was the original channel of the San Gabriel River. In fact, where the Rio Hondo joins the Los Angeles River today, the L.A. River actually lost its name and became the San Gabriel.
During the heavy winter floods of 1861-1862 the San Gabriel River overflowed its banks and created a seperate channel (Then called the "New River"), this new path let out at Seal Beach and is the path of the San Gabriel we know today. The river's former channel became known as the Rio Hondo. They're all ugly flood control channels now too.