thanks Cranky!
okay.. so the bikes were found across the lake from the docks but the dogs traced them to the dock (per the aunt which was posted upstream)
the dock shown is the fishing dock, I believe and is fixed. the bikes would be on the other side of the lake. but again, the lake is not huge so it isn't real far. but if the girls were going to swim from the dock, why leave your bikes on the other side. the nature trail is wooded in some areas. Iowa has been working on establishing connecting nature/bike trails across the state and most run through parks, wooded areas, along lakes and cornfiels.
They could have been "exploring" and ended up on the other side of the lake, too. Bushwack or be on a "lion hunt" to the other side?