This happened when river water regularly deposited sediments over land areas during the river's annual floods, accumulating thick layers of sand, mud, and silt.
(This; happened; T:when river water regularly deposited sediments over land areas during the river's annual floods, accumulating thick layers of sand, mud, and silt) (river water; regularly deposited; sediments over land areas; T:during the river's annual floods; accumulating thick layers of sand, mud, and silt)
This river water
happened regularly deposited
T:when river water regularly deposited sediments over land areas during the river's annual floods, accumulating thick layers of sand, mud, and silt
sediments over land areas T:during the river's annual floods accumulating thick layers of sand, mud, and silt