Despite reductions in acidic deposition, high freshwater Al concentrations continue to threaten acidified ecosystems across the northern hemisphere. Seasonally elevated Al concentrations may pose a particular threat to freshwater organisms. Despite this threat, there is a lack of understanding about the timing and drivers of seasonal Al fluctuations. Here, we address this knowledge gap by identifying seasonal patterns of Al and their drivers in 16 rivers across Nova Scotia, Canada. We identify five distinct Al regimes with different timing of seasonally elevated Al concentrations. Regimes are distinguished by correlation strength and direction between Al and base cations, total organic carbon, turbidity, and discharge. Most notably, regimes are distinguished by a gradient of Al-base cation decoupling as Ca and Mg concentration approaches 1.4 mg L-1 and 0.6 mg L-1, respectively. Seasonally elevated Al concentrations exceeded the 0.1-0.2 mg L-1 World Health Organization drinking water guidelines in all regimes, and inorganic monomeric Al is projected to exceed the 15 μg L-1 threshold for aquatic health in most rivers. This research highlights the complexity of seasonal Al dynamics and the importance of understanding seasonal variation of Al to quantify the impact of Al on human health, water treatment, and aquatic organisms.