Many of these issues have come into sharp focus in Lyons, a small community of 2,000 situated between Boulder and Estes Park, Colo., known for bluegrass and folk-music festivals.
The two branches of the St. Vrain meet in Lyons, but in many places, they no longer flow where they did. At one point in town, a bridge now spans a dry riverbed. Just in front of it, the river flows through a stretch of obliterated highway. A new creek cuts through Park Street, hampering access for Tiller and other residents, whose flooded homes lie beyond it.
In other areas, the river is running in its natural channel, but has also split off to run in parallel channels.
Moreover, the creeks cut enormous new channels through normally dry areas that are a concern for future flooding, or even for usual spring runoff, engineers say.
"The obvious thing is to put [the river] back where it was," says Scott Shipley, an engineer from Lyons who designs white-water parks and is helping the town with its current decisions. Leaving the river in its new channels isn't necessarily safer, Mr. Shipley says, and in many cases produces major complications involving water rights, ditch rights, and property rights.
But restoring the river to its former path could be tricky – and expensive. And to the extent that engineers can find places to leave buffers – places where the river can spread out into wetlands and flood plains, alleviating pressure downstream – it will be better for future flooding, Shipley says.
"Part of what we're trying to do is build this town back better than it was," he says.
At this point, essentially the entire town of Lyons is evacuated since all utilities – water, sewage, electricity, gas – were lost in the flood. The town's public works building is in shambles, and weeks after the flood, the city was still trying to locate water mains.
Power and gas have now been restored in some areas, so that some businesses can resume minimal operations, but the town has warned that the businesses assume risk for any fires.