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Climate change could impair navigation by changing average water levels in rivers and lakes, increasing the frequency of both floods during which navigation is hazardous and droughts during which passage is difficult, and necessitating changes in navigational infrastructure. On the other hand, warmer temperatures could extend the ice-free season.
 The elaborate system of locks and dams that smooth seasonal cycles on major inland waterways was originally built primarily to reduce or eliminate shoals and rapids, and to maintain flow despite fluctuations in runoff and other climate variables. Fortunately, the projected changes in monthly mean climate are modest compared to the inter-annual variability that the system was designed to handle. Therefore, major direct threats to navigation are unlikely, as long as federal law continues to grant navigation a priority in the management of the locks and dams. If other priorities such as water supplies were threatened, statutes might be modified. Moreover, if the year-to-year variability of climate were to change, the threat to navigation might become more severe.
The modest increase in flooding from the increased annual rainfall and somewhat more intense rainstorms would not, by itself, threaten the federal navigation infrastructure. On the other hand, the unprecedented 1993 Mississippi/Missouri River flooding shows that a wetter climate could overwhelm the federal infrastructure; navigation on the river was suspended at flood stages lower than those that eventually caused most of the flood-related damages, even before inland flooding became a problem. Unfortunately, no one has established whether droughts or severe floods are likely to be more or less frequent. Another concern for navigation in the lower Mississippi river is that rising sea level could directly or indirectly impair the ability of the Corps of Engineers to maintain the shipping lanes south of New Orleans.
Climate change seems likely to have both positive and negative impacts on Great Lakes shipping. The expected decline in Lake levels would reduce the flow and hence the depth of the St. Lawrence River; reducing the tonnage per ship and increasing shipping costs 1 percent for every 5 cm drop in lake levels. On the other hand, warmer temperatures could extend the ice-free season by aa much as three months in Lake Erie.
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