[jamest (OP)]

Joanne wants to know why our interconnected water system varies so greatly in salt content.

[paul cotter]

Fresh water bodies are supplied by precipitation which is close to pure water with negligible salt content. Rivers run into oceans carrying small quantities of various salts leached out from rocks. Water evaporates from the oceans leaving whatever salts are present in the oceans and the evaporated water then becomes precipitation to start the cycle over again. Over time this process has concentrated the salt in the oceans.