Those big rounded cobbles down at the base of the embankment are called alluvium.  Whereas colluvium consists of angular cobbles that have not been subject to rounding, alluvium has been transported by streams for a long-enough distance to knock off the angular edges.

Although the alluvium is part of the City of Tempe's "landscaping",  these rounded cobbles were actually taken from this spot, long before there was a Rio Salado street or a Tempe Town Lake. They came from the Salt River.