Before the latter 19th century, Santiago Creek and its tributaries were free flowing perennial streams spilling out of Santa Ana Mountains canyons onto the broad, alluvial floodplain. The creek wound on the plain for the rest of its course to its confluence with the Santa Ana River. coast live oaks (Quercus agrifolia), California sycamores (Platanus racemosa), white alders (Alnus rhombifolia), native willows (Salix species), and other riparian habitat vegetation lined the route of the creek and its primary tributaries. The creek and adjacent habitats supported a wide variety of birds, amphibians, fish, insects, and mammals.