The Santa Ana River rises in the southern San Bernardino Mountains, at the confluence of two tiny streams, Heart Bar Creek and Coon Creek, at an elevation of . Its highest sources are Dollar Lake, at , and Dry Lake, at , both on the northern flank of San Gorgonio Mountain, at the headwaters of the South Fork Santa Ana River. The river flows west through a wide, deep and heavily forested mountain valley. About from its headwaters, it receives its first major tributary, Bear Creek, which enters from the north. Bear Creek receives its water from Big Bear Lake, a popular recreational mountain lake. The river turns south, passing through the Seven Oaks Dam, and reaches the arid Inland Empire lowland covering large parts of San Bernardino County and Riverside County. It receives Mill Creek from the south and passes to the south of San Bernardino, then receives City Creek from the north and San Timoteo Creek from the south. Due to water diversions for groundwater recharge, the river bed is usually dry in this stretch between Mill Creek and the outlet of the Veolia water treatment plant north of Riverside, which restores a year-round flow. From there to Prado Dam the river supports a riparian zone with considerable greenery.