THIRTY-YEAR WATER QUALITY TRENDS IN HIGH ROCK LAKE, NORTH CAROLINA

Thirty-year water-quality constituent records of dissolved oxygen (D.O.), turbidity, fecal coliform bacteria, total suspended solids (TSS), water temperature and metals from four North Carolina Division of Water Quality ambient monitoring stations unveiled long-term environmental change in High Rock Lake, an impoundment of the Yadin River and its surrounding watersheds. The results are as following: (1) D.O. rose in the early 1970s in the lake, began to fall in the middle 1980s, reached its lowest trough at the beginning of 1990s, and has been rising since. (2) Fecal coliform concentration drastically declined during the 1970s. Since the turn of 1980s the overall trend remains stable lake-wide. (3) The TSS trend has declined lake-wide since the end of the 1960s. However, the trend in the behind-dam watershed has shown a slight climbing in the last decade. (4) The turbidity trend has been declining lake-wide. However, the declining rate varies: the rate in the headwater and mid-lake areas is slower than that in the behind-dam watershed. Discrepancies between TSS trend and turbidity trend in the lake and the behind-dam watershed implies different biochemical regimes for these water bodies. (5) The water temperature trend declined during the 1970s and early 1980s, then began to rise, peaking in the mid- to late 1980s, and have been declining since. To identify long-term environmental enforcing mechanism and its frequency regimes that act upon the lake system, a time-series analysis was carried out using the same water-quality records and, in addition, a 60-year-long USGS daily flow record from the Yadkin River. It is noted that, beside the predominant annual-cycle component, there are at least two other major cyclic components impinging upon the system: the 3~4-year and 12~16-year cyclicities. It is postulated here that the former is related to the global-wide El Nino and La Nina weather event, the later, however, remains to be investigated.