Background/Question/Methods By 1950, vast acreages of semi-arid Colorado were irrigated by about 17,000 miles of ramifying canals. Today, historic canals increasingly cut through urban and suburban areas, with associated land use practices shaping both vegetation and soil characteristics. Given their proximity to population centers, some earthen canals are being reimagined as green stormwater infrastructure (GSI) that can be co-opted to hold, infiltrate, and clean incoming stormwater. As with traditionally constructed (i.e., planted and soil-amended) wetlands, the in situ vegetation and soil of repurposed canals will largely shape their efficacy in a stormwater context. However, while there is a growing interest in using existing canals to treat stormwater, it is unclear how these uniquely managed waterways might function in this new capacity. Here, we assessed plant community and soil characteristics along a one-mile pilot reach of the High Line Canal near Denver, Colorado, which is slated to receive and hold water for up to 100 additional days per year. Results/Conclusions We found that sod-forming, rhizomatous, perennial graminoids were the dominant functional group, followed by deep-rooted shrubs, suggesting existing capacity to stabilize the canal banks, reduce erosion, and promote water infiltration into the soil under increased flooding. Obligate and facultative wetland species, expected to be comparatively resilient to increased flooding versus upland species, were patchily distributed (restricted to the lower canal banks) and not particularly abundant across the reach (<30% cover), indicating a potential compositional and functional shift in the plant community over time. Related, we found that two-thirds of the native species pool was composed of water-loving to mesic species, while two-thirds of the non-native species pool was composed of upland species. The native species pool might thus prove more resilient to the impending hydrologic shift. Despite their urban setting, soils did not appear degraded, with no evidence of compaction (bulk density range of 1.19-1.31 g/cm3), depleted soil organic matter (range of 4.1-8.9% in predominantly sandy loam soils), or high salinity (using electrical conductivity as a proxy; range of 0.44-2.04 mmhos/cm). Soils are thus likely to support needed vegetative cover and water infiltration into the soil. Taken together these findings suggest the canal reach may well mimic the functionality of traditionally planted and soil-amended GSI.
