By catching rain in a barrel, you will have soft water for your rain garden or anything else you would like to use it for. This may prevent soil from washing away.
A rain garden should be located where runoff can be diverted into it away from building foundations and utilities. Plant perennial plants native to the Great Lakes region. Mulch of shredded hardwood will keep the soil moist.
For more information about how to make a rain garden go to http://www.raingardens.org/
Rain gardens replenish groundwater supplies and when the water is filtered by plants it will be clean when it enters lakes and streams.