A) Mountains and hills sometimes resemble enormous dragons. In fact, In China there are many mountains that are named after Dragons because they have uncanny resemblances to dragon faces, tails, horns, and backs. Unwary travelers could easily see these as dragons:
B) Swamps and marshes produce an amazing amount of methane gas and oxygen because of the enormous amount of algae. They are notorious for small flare ups and bursts of fire that come from these gases mixing together. Perhaps if travelers came across a swamp and suddenly saw a burst of flames shoot up from the marsh, they would think it was a dragon warning them to stay away.
C) In deep rivers, natural whirlpools are extremely dangerous and feared by ships and canoes. There are dozens of Native American myths that revolve around the whirlpool(pun intended). A few of them relate to dragons, they believe that whirlpools are caused by dragons and that if you stare into one, you will immediately drown. To avoid this, Native Americans would offer sacrifices to rivers and even temporarily blind themselves with peppers in order to avoid staring into these deadly whirlpools.
). Its not too hard to imagine that later map makers took the idea and simplified into smaller sea serpents.
