I think it is an error : the water of the river does not "disappear", it merges with the canal then goes outside the canal : the parts of the canal should be part of the relation for the 2 relations Eider and Giselau (like when a river goes through a lake, the water merges with the lake, there is no more river but we connect the entrance in the lake and the exit of the lake with a way in the middle of the lake, a false way required only to have a waterway with no gap).