Bioswales are storm water runoff conveyance systems that provide an alternative to storm sewers. They can absorb low flows or carry runoff from heavy rains and snowmelt to storm sewer inlets or directly to surface waters. Bioswales improve water quality by enhancing infiltration of the first flush of storm water runoff and filtering the large storm flows they convey. About 30 to 40 percent of Montana’s annual precipitation comes from small rain events. Much of the value of bioswales comes from filtering nearly all of this water.