Water flowing in the San Gabriel River come from rains of the past and from snows higher and farther away. Water slips through cracks in stones and through tiny grains of stone from deep within... jumping out at the sides of the hills... falling to the ground... darting here and then disappearing there.... Most years there is very little water. Plants have evolved to reach deep in the ground or come out of their seeds at wet times or after a summer fire or even a winter mud slide/flash flood. Alders, like willows need more water and so crowd the river's edge. Animals get water from where ever they can... some come down the mountains to the houses and pools... and streets.
This river flow is at the mid section of the mountain high above Pasadena, California.... At one time tiger beetles ran the river's mud banks. Are there any left?