I wonder if this was built to accommodate the old narrow gauge railroad that used to run nearby. The old roundhouse for it still stands a couple of blocks away. Seems odd for such a structure to be built specifically for a creek that barely flows and could essentially run off towards river without affecting the integrity of the works above...
The attached 1900 map shows a spur of the Burlington, Cedar Rapids & Northern crossing Ralston to the south at Lafeyette Street, and the Rock Island bridging only Ralston.
Map source: http://www.historicmapworks.com/Map/US/153978/Iowa+City+3/Jo...
Also Ralston Creek was historically known to flood pretty badly. For example, the Great Flood of 1881.
https://books.google.com/books?id=BfIN2L5htEwC&pg=PA444&dq=j...