WISCONSINPhotos: Mississippi River lock and dam system is critical to economySite safety and health officer/deckhand Dan Burger stands on the lock gates as they swing open for a northbound towboat with barges at Lock and Dam No. 7 on the Mississippi River between Onalaska and La Crescent, Minn. A University of Wisconsin-Madison study shows most of the lock-and-dam infrastructure on the Upper Mississippi is nearly 100 years old. Maintenance on it is behind schedule, with an estimated backlog of more than $1 billion.Mark Hoffman / Milwaukee Journal SentinelSite safety and health officer/deckhand Dan Burger watches turbulent water pass through a roller gate on the downstream side at Lock and Dam No. 7 on the Mississippi River between Onalaska and La Crescent, Minn.Mark Hoffman / Milwaukee Journal SentinelU.S. Army Corps of Engineers maintenance equipment is shown tied up at Lock and Dam No. 7 on the Mississippi River between Onalaska and La Crescent, Minn.Mark Hoffman / Milwaukee Journal SentinelJim Rand, chief of locks and dams, looks at the gearing installed in the mid-1930s that controls an 86-foot-wide roller gate on the dam at Lock and Dam No. 7.Mark Hoffman / Milwaukee Journal SentinelThis is part of the gearing installed in the mid-1930s that controls an 86-foot-wide roller gate on the dam at Lock and Dam No. 7 on the Mississippi River between Onalaska and La Crescent, Minn.Mark Hoffman / Milwaukee Journal SentinelDeckhands from the Britney Lee lash together barges at Lock and Dam No. 7 on the Mississippi River.Mark Hoffman / Milwaukee Journal SentinelThe Britney Lee pushes barges after lockage upstream at Lock and Dam No. 7 on the Mississippi River.Mark Hoffman / Milwaukee Journal SentinelThis auxiliary sheet pile closure (foreground, left) at Lock and Dam No. 7 was constructed because the auxiliary lock gates (rear, left) cannot be replaced. Without the sheet pile closure, if the old lock gates were to fail, the lock would be dewatered.Mark Hoffman / Milwaukee Journal SentinelFishermen find the downstream side of the lock and dam is a prime spot.Mark Hoffman / Milwaukee Journal SentinelThe tug Penny Eckstein enters the lock while traveling northbound at Lock and Dam No. 7.Mark Hoffman / Milwaukee Journal SentinelTurbulent water passes through a roller gate on the downstream side of Lock and Dam No. 7.Mark Hoffman / Milwaukee Journal SentinelThe Best Dam Fishing Float is a for-profit spot for anglers where fish congregate on the downstream side of Lock and Dam No. 7.Mark Hoffman / Milwaukee Journal Sentinel