It is thus by sub-glacial aggradation that the prairies have been leyelled up to a smooth surface, in contrast to the higher and non-glaciated hilly country next south.
The floor of this depression being below baselevel, it has necessarily come to be the seat of the mountain waste brought down by the many streams from the newly uplifted Sierra Nevada on the east and the coast ranges on the west; each stream forms an alluvial fan of very gentle slope; the fans all become laterally confluent, and incline very gently forward to meet in a nearly level axial belt, where the trunk riversthe Sacramento from the north and the San Joaquin from the south-east--wander in braided courses; their tendency to aggradation having been increased in the last half century by the gravels from gold washing; their waters entering San Francisco Bay.
Flood plains may be the result of planation, with aggradation, that is, they may be due to a graded river working in meanders from side to side, widening its valley by this process and covering the widened valley with sediment.