Sentence:  In real estuaries, river water and seawater seldom mix together directly as <WSD>endmembers</WSD>. Rather, mixing takes place in a continuous manner that may be approximated by a series of water types, each of which has a salinity close to that of its neighbours. 
Meaning of the word marked with <WSD> tags:  A member at one end of a range or series made up of similar members.