The effects of inland evaporation are aggravated. Gradually, the repletion level of local fresh-water lakes, (left behind thousands of years ago in basins, during periods of heavy rain, or the result of melting mineral-laden glaciers at the end of the last Ice Age), is lowered.
I removed the conjunction "and" created two new sentences for easier comprehension. I also added a few commas to make the sentence easier to read. I only used the brackets to show the extra information that is given so that it is easier to identify the subject of "is lowered". Now, re-read the sentence without the words in the brackets; you can identify the subject as being "the repletion level".
I hope that this answered your question.