This is as close to the headwaters of the Arroyo Seco – where the river begins – as we can get. Down in Colby Canyon, below, is where the Arroyo Seco begins it’s journey out of the mountains. Rain and melting snow are shed off of the canyon walls and collect at the base of the canyon before flowing downstream. The east-west orientation of the canyon is due to the San Gabriel Fault = the approximate trace of the fault is shown on the map below in red). Movement along the fault caused rapid erosion of the fractured rock along the fault created this alignment in addition to the deep V shape of the canyon. Canyons in mountainous areas often form this V shape, as the river is trying to cut down through the bedrock towards base level – the elevation of sea level which is, in this case, the Pacific Ocean.