The desert country toward the mouth is largely a sandy or gravelly aggradation plain of the river.
"Encyclopaedia Britannica, 11th Edition, Volume 6, Slice 6" by Various
Its deposits tend to aggrade (build up) the flat which its meanderings develop.
"The Geography of the Region about Devils Lake and the Dalles of the Wisconsin" by Rollin D. Salisbury
In this way each part of the fan is aggraded and its symmetric form is preserved.
"The Elements of Geology" by William Harmon Norton