That's a thimble: When I zoomed out on the map the airport was huge and the river just a few pixels wide. Otherwise I wouldn't have made the comment I made.
Airports are far larger than aircraft, which enter and leave from the end of runways.
Rivers have a nice property where they tend to be the lowest thing around because water flows downhill and are quite long. For approaches from other directions half the time you end up crossing the river at some angle which acts as a buffer zone, thus reducing the total land lost as a buffer zone.
