Most lakes and ponds have streams flowing into and out of them. Body of waters all tend to flow together. Think of the vast network of rivers, streams, brooks, lakes, and ponds that all flow into the mississippi. While the water all runs in one direction, fish can of course swim up current.

Even when bodies of water are not directly connected, when there's a huge rainfall and floods in one area that water carries fish for miles and miles. There can be divides where normally all water flows eastward, but when there's a flood at a divide, water can flow out both directions, and species that only had access one way find themselves in a different interconnected network.