Monte-Carlo simulations were used to assess the short-term ANC depression of Sierra Nevada lakes due to acidic deposition events. The Episodic Event Model (EEM) was used to simulate snowmelt events as well as the summer dry deposition and rainfall events. The model assumes that during events, there are no reactions occurring in the watershed which would neutralize the incoming acidity entering the lake. Consequently, the results of this study represents the worst case scenario. The parameters of the EEM model was derived from available databases. Spring snowmelt events were shown to have greater impacts on the water quality of the Sierran lakes than summer events. Under annual average loading conditions, no lake in Sierra Nevada is acidic although 29% of the lakes have ANC less than 40 peg/L. During early snowmelt events simulated using present H+ loading conditions, 79% _+ 9% of the lakes will experience short-term ANC depressions to levels less than 40 peg/L. The summer event simulations indicate that under present H+ loading conditions, 31% of the lakes will have a short-term ANC depression to levels less than 40 peg/L. The most critical parameters which control the magnitude of the ANC depressions during both, snowmelt and summer critical events are a) the lake area to watershed area ratio and b) the volume of water in the mixing zone.