Author: Bernadette Kuhn
Contributed by Bernadette Kuhn, Colorado Natural Heritage Program
Denver County’s urban wetlands provide critical wetland functions such as wildlife habitat, storm water and sediment retention, and groundwater recharge in an otherwise arid and developed landscape. They provide these benefits despite their highly modified hydrology and lack of large buffers. These wetlands also provide a range of recreational and educational opportunities for Denver’s urban community.
In 2013 and 2014, the Colorado Natural Heritage Program (CNHP) conducted 40 wetland condition assessments in Denver County using the Ecological Integrity Assessment (EIA) methodology for Colorado wetlands (Lemly and Rocchio 2009; Lemly et al. 2011). Additionally, CNHP updated National Wetland Inventory (NWI) mapping for Denver County. The full report and an education brochure are available on the CNHP website. The goal of this study was to document the condition and extent of wetlands in Denver County. Although other investigators have examined wetland condition in the South Platte Basin, including the Cherry Creek Dam (Cooper 1989), the northern Front Range (Lemly et al. 2012), and the South Platte River basins (Lemly et al. 2014), none have specifically examined urban wetlands in the heart of Denver.
Results from our wetland mapping indicate that wetlands (including waterbodies) are uncommon in Denver County and account for 2.5% of the land area. The majority of acres mapped in the NWI are large constructed water storage reservoirs, water conveyance canals, and natural rivers that dot the landscape in Denver County. These waterbodies provide surface water storage, sediment retention, groundwater recharge, and aquatic habitat. Vegetated wetlands and small ponds represent only 0.7% of the land area, but they provide nutrient cycling, shoreline stabilization, biodiversity support, native plant community maintenance, and terrestrial habitat function.
The EIA framework used for the condition assessments evaluates wetland condition based on four biotic and abiotic categories: 1) Landscape context, 2) Biotic condition, 3) Hydrologic condition, and 4) Physiochemical condition. Each category contains three to six metrics, which are used to evaluate how far the wetland deviates from reference condition (i.e., before human disturbance). Both qualitative and quantitative criteria are used to score each metric. The metric scores are then consolidated into a category score, and category scores are combined into an overall EIA score and rank. Possible scores range from 1.0 (worst) to 5.0 (best), and can be assigned alphabetic ranks of A (best), B, C or D (worst), which correspond to different levels of alteration and represent different management opportunities. Out of 40 urban wetlands in our study, only one scored in the C range. The remaining 39 scored D ranks.
Low ranks for Denver’s urban wetlands are due to high levels of human disturbance and altered hydrologic processes. These scores reflect the need for improving Denver’s wetlands. CNHP’s primary management recommendations are:
1) allow more natural vegetation to grow along existing waterbodies and wetlands by reducing mowing zones, bare surfaces, and manicured lawns;
2) reduce or eliminate the use of herbicide, pesticide, and fertilizer treatments in or near wetlands to protect water quality and wetland habitats;
3) reconnect rivers and creeks to their floodplains and facilitate structural diversity in wetlands adjacent to rivers to improve or restore a variety of wetland functions.
All 40 wetlands contained relatively high biodiversity given their urban context, as well as plant species previously undocumented for Denver County. Plant diversity ranged from 19 taxa to 128 taxa across all urban wetlands, with an average of 62. A total of 24 plant taxa were documented during our study that had not previously been reported for Denver County. The study also documented three rare plant species: sweet flag (Acorus calamus), prairie ragweed (Ambrosia linearis) and broadfruit bur-reed (Sparganium eurycarpum).
The final report and appendices contain full plant species lists for all 40 of the urban wetlands, as well as bird lists, information on functional acreages in Denver’s wetlands (i.e. how many wetland acres are high vs. moderate vs. low functioning for groundwater recharge, nutrient cycling, etc.), wetland types, wetlands by ecological system, full site descriptions and maps of each assessed wetlands, noxious weed lists by site, county plant records, and more management recommendations. More information about Colorado’s wetland resources, as well as interactive wetland mapping tools, can be found at the Colorado Wetland Information Center website.
References:
Lemly, J. and J. Rocchio. 2009. Field testing of the subalpine-montane riparian shrublands Ecological Integrity Assessment (EIA) in the Blue River watershed, Colorado. Colorado Natural Heritage Program, Colorado State University, Fort Collins, CO.
Lemly, J., L. Gillian, and M. Fink. 2011. Statewide strategies to improve effectiveness in protecting and restoring Colorado’s wetland resource. Colorado Natural Heritage Program, Colorado State University, Fort Collins, CO