A lot ofvwater ways and watery connections are gone due to ditching out mosses and marschlands. And some waterways have grown over in bogs and swamps. If one look at older maps one can often see connections between lakes not seen today. Only in Sweden 6000 lakes and ponds are gone since the beginning of the 20th century.

I don't know about this idea of mine but might there be older cultures planting fishes in lakes for future fishing?