What separates Upo from other bodies of water is that it was left rather than made. As the Nakdong River shifted its channel over time, water cut off from the main stream settled into low ground and stayed. No embankment holds it — this is where the river used to run — so the sediment beneath and the ecosystem above it formed over a very long span. That is why the phrase "a living natural history museum" attaches to the place so often.
A marsh is not a lake. The water is shallow enough for light to reach the bed, so plants grow underwater, and when water plants tangle root and stem into mats they read as islands of grass floating on the surface. That shallowness is the condition for the biodiversity. Fish, aquatic insects and water plants layer into one another with birds settling on top, so an equivalent area of deep lake holds far less life.
It becomes a different place by season. Spring and summer green the surface with water plants; autumn rings the edges with reeds and silver grass; winter opens the water and brings a large increase in migratory birds. If birdwatching is the point, winter is overwhelmingly better; if the scenery is, early morning when mist rises off the water is the time. Marshes have a wide morning temperature gap and fog often forms — that image is the one most people know Upo by.
The trail runs around the edge and circling the whole thing takes a while. You can rent an electric bicycle, or simply walk one section. Visiting the ecology centre first, to see how the landform came about, changes how you read the shape of the water afterwards. Since protection of wildlife comes first here, staying off the water's edge and keeping noise down are basic courtesy — more so in winter when the migratory birds are in.