Sentence:  In many of the vast sea-level swamps of the Atlantic side, no slope whatever can be obtained, and here dependence is placed on a grid system of intercepting earthen ditches, communicating with seawater at each end whenever practicable, to insure quick run-off of rain water and thorough <WSD>salination</WSD> of every part of the swamps. 
Meaning of the word marked with <WSD> tags:  The increase of salt content in water.