The division heretofore made by the Legislature of the alluvial land of the state into two levee districts — viz., the Yazoo-Mississippi Delta Levee District and the Mississippi Levee District, as shown by the laws creating the same, and the amendments thereto — is hereby recognized, and said districts shall so remain until changed by law; but the legislature may hereafter add to either of said districts any other alluvial land in the state.