Abstract.Ocean acidification threatens to reduce pH and aragonite saturation state (Ω A ) in estuaries, potentially damaging their ecosystems. However, the impact of highly variable river total alkalinity (TA) and dissolved inorganic carbon (DIC) on pHand Ω A in these estuaries is unknown. We assess the sensitivity of estuarine surface pH and Ω A to river chemistry using a 1-dimensional, biogeochemical-coupled model of the Strait of Georgia on the Canadian Pacific coast and generalize the results 5 in the context of global rivers. The productive Strait of Georgia estuary has a large, seasonally variable freshwater input from the glacially fed, undammed Fraser River. Analyzing TA and pH observations from this river and its estuary, we find that the Fraser is moderately alkaline (TA 500-1350 µmol kg −1 ) but relatively DIC-rich, especially during winter (low flow). Model results show that estuarine pH and Ω A , while sensitive to freshwater DIC and TA, do not vary in synchrony. Instead, rivers with high DIC and TA produce lower estuarine pH due to an increased estuarine DIC:TA ratio, but higher estuarine Ω A because of 10 DIC contributions to the carbonate ion. This estuarine pH sensitivity decreases with increasing mean river TA, but the zone of maximum pH sensitivity also moves to higher salinity which could impact a larger areal extent of the estuary. Many temperate rivers, such as the Fraser, are expected to experience weaker freshets and stronger winter flows under climate change, reducing the extent of the river plume and the impact of river chemistry in much of the estuary. However, increasing carbon in rivers will move the highest sensitivity zone to higher salinities that cover larger areas under present-day flow regimes.