Major river systems and associated Quaternary and Holocene sediments. Except for the Brazos and Colorado, all of the rivers terminate in bays that were formed by flooding of the lower river valleys by the 400 foot rise in sea level that occurred during the past 20,000 years. The Brazos and Colorado Rivers transported large volumes of sediment, which filled the lower river valleys and prevented marine flooding. Only very shallow water bodies, such as Christmas Bay and Cedar Lakes, occur in the vicinity of this delta.