Does a Large On-Line Stormwater Pond put the Heat on the Downstream Environment

Increases in downstream water temperatures and potential effects on stream life as a result of on-line stormwater ponds is an issue that is currently receiving some attention. Most of the research experience on such temperature effects has been around existing small ponds with no New Zealand before and after case study examples.

CDL Land New Zealand Ltd (CDL) is developing a 74 ha residential subdivision in the growth area of Flagstaff in northern Hamilton. The development includes a large on-line stormwater detention pond (1.23 Ha) constructed over the bed of the main catchment stream. The project offered a valuable opportunity to gather water temperature data before and after lake construction.

Stream temperature data were collected from stream sites upstream and downstream of the lake footprint over two summer periods prior to lake construction and over two summer periods following construction. Monitoring captured likely worst case scenario conditions during the 2008 drought prior to the lake and the 2012/2013 drought post construction. The data show a clear increase in peak summer water temperatures in the downstream environment following lake construction. However, the frequency that effect thresholds for native stream fauna have been exceeded was less than anticipated.