alluvial       
adjective uk/əˈluː.vi.əl/ us/əˈluː.vi.əl/
 冲积的,淤积的
made up of sand and earth left by rivers, floods, etc.
  an alluvial plain 冲积平原 
 Some alluvial deposits are a rich source of diamonds. 一些冲积矿床富含钻石。 
 See: alluvium
 - More examples
 - Many crops grow well in the rich alluvial soil.
 - Settlers built their homes on the fertile alluvial land.
 - The country's alluvial diamond fields are some of the richest in the world.