Research Article published by HindawiThis study reports the variations of fluoride ions in rivers on the slopes of Mount Meru in the northern part of Tanzania. More than 318 water samples were collected from Temi, Nduruma, Tengeru, andMaji ya Chai Rivers in both wet (mid-March and April) and dry (August) seasons. The samples were analyzed for fluoride levels using Ion Selective Electrode (ISE). The minimum and maximum average fluoride levels in the wet season were 0.24 ± 0.03 mg/l and 65.20 ± 0.03mg/l, respectively, whereas the average lowest and highest levels in the dry season were 1.02 ± 0.02 mg/l and 69.01 ± 0.03mg/l, respectively. Tengeru River had the lowest fluoride levels in both seasons, whereasMaji ya Chai recorded the highest fluoride levels in both seasons.Theheadwater of all rivers with the exception of Maji ya Chai met theWorld Health Organization’s (WHO) maximum acceptable fluoride levels of 1.50mg/l and the downstream environment qualified for Tanzania Bureau of Standards (TBS) maximum permissible fluoride concentration in drinking water of 4.00mg/l. Also, the laboratory experiments showed that fluoride containing rocks exposed to pH above 7.6 display high leaching of F− in solution which gradually increased with the increase in pH, indicating that dissolution of fluoride in water is a function of pH.