My biggest brook trout was caught out of a stream no wider than 24". Didn't even need to jump over the stream. After pulling it in I realized the stream was much wider than I had thought. I layed down on my belly, stuck my whole arm into the shallow stream, and couldn't tell how wide the stream was beneath me. I was standing on about 18" of land suspended over the water. Apparently this is common for streams to cut away walls but the vegetation holds on to the top layer of soil.
When fishing, you never know till you try.