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islet
Basin
An island is land surounded by ocean -Ali Al-Wazzan aka wolf
A divide
I once thought that too. Not, not all land is a island. A island is a small peice of land, a contry or continent is a big peice of land. - But they're surounded by water!
all surrounded is called an island but if its only almost then its a peninsula
bay
the gradient and how much friction there was. The gradient means how steep the land the river is on so if it is very steep them the velocity will be higher.
A valley.
land line
Given the Nile is over 4,000 miles long the only answer to your broad question is - in places it is lower and in places higher.
My thoughts are: Land is to River as Body is to Artery or veins~ perhaps?