Evaluating the impact of culvert designs on hydrologic connectivity and nutrient uptake in Northern Wisconsin streams
Stream restoration projects have become increasingly common in recent years, but typically lack post-restoration monitoring to evaluate whether projects meet expectations or impact ecosystem functions. The USDA Forest Service has worked to improve designs of culverts that are replaced to restore aquatic organism passage, with the assumption that ecosystem functions will also be better maintained. One such design is the stream simulation culvert, where the streambed and channel through the culvert are rebuilt to mimic natural width, depth and velocity. Our objective was to investigate whether stream simulation designs maintain ecosystem functions better than other designs by evaluating two measures of ecosystem function: nutrient cycling and hydrologic connectivity. We selected three stream simulation culverts and paired each with a recently replaced, non-stream simulation culvert. Ammonium uptake was measured in reaches (50-250 m long) above and below culverts using standard stream nutrient spiraling techniques. Salt pulses were completed above, through, and below culverts to determine travel times and to model transient storage. Nutrient uptake and hydrologic characteristics were then compared using t-tests with the expectation that reaches would be more similar above and below at simulation compared to non-simulation culverts.
Nutrient uptake velocities ranged from 0.042 to 0.091 mm/sec, with no differences above versus below or between different culvert designs. Mean difference in travel times between above and below culvert reaches at stream simulation sites was 1.9 sec/m compared to 2.5 sec/m for non-stream simulation sites. Mean travel time through non-stream simulation culverts was 75.8 sec/m, which was much longer than the mean of 15.6 sec/m for above and below reaches of all non-stream simulation culverts. Mean travel time through stream simulation culverts was 14.6 sec/m and 10.9 sec/m for above and below reaches of all stream simulation culverts. There was no statistical difference for travel times of different culvert designs or between above and below reaches, but we were only able to sample 2 culverts pairs due to low water conditions. These results suggest that stream simulation culverts do not impact stream nutrient spiraling or hydrologic characteristics in upstream and downstream reaches differently than other culvert designs, but that hydrologic characteristics through the culverts may be very different between two culvert designs. Further investigation of transient storage characteristics with additional replication should allow for more robust analysis of culvert design effects.