Chatfield Reservoir, in Littleton, CO, provides municipal drinking water, recreation, and flood control to the Denver Metropolitan area. Implementation of the Chatfield Storage Reallocation Project, proposed by the Army Corps of Engineers to increase water storage capacity, began in Fall 2017. The reallocation plan will result in increased flooding accompanied by planned removal of large dead snags and other major modifications to cottonwood forest communities around the reservoir and its tributaries. These cottonwood forests occur in two distinct age categories: 1) younger stands in dense formation, and 2) more widely spaced older legacy trees. The reservoir and adjacent cottonwood forests are also an important destination for seasonal, migrant, and resident bird species. This study is in two parts: 1) Comparative study of bird usage and stem density in the two forest types, and 2) Before/After/Control/Impact (BACI) study to evaluate the effects of periodic flooding on forest habitat and bird occupancy. Bird point counts were done in each forest type during spring and fall migratory seasons of 2016 and 2017, before flooding begins, and at control sites in fall 2017. Vegetation surveys were conducted according to a modified BBIRD protocol at the bird count sites during those same years.
Results/Conclusions
The number of small trees (<38 cm) is significantly greater in stand forest (18.22±10.78 stems, n=10) than in legacy forest sites (8.15±5.86 stems, n=9), according to the Wilcoxon rank-sum test (W=24.5, p=0.025). Despite these differences, bird species diversity does not appear to differ significantly between stand and legacy sites, according to Shannon’s beta index (ß=1.10 for Spring 2017 and Dß=1.56 for Fall 2017). It is possible that bird diversity is not affected by differences in stem density, or it may be that bird usage is correlated with stem density on a more granular scale than legacy versus stand comparisons imply. By comparing vegetation composition to bird usage, we may be able to anticipate likely effects of reservoir level changes on avian diversity at Chatfield.