Macropore flow at the groundwater/surface water interface to a humic lake and the seepage fluxes through the lake bottom were measured, as well as was the groundwater chemistry. With a 100-fold difference in discharge from the lake, the seepage rates through the lake bottom differed by only 15%, indicating that macropores provided a major part of the inflow during high-flow events. Two larger macropores had flow rates between 0.3 and 0.7 L s-1, corresponding to velocities of 1--3 m min-1. The flow rates of four smaller macropores were recorded to only 0.02--0.1¿10-4 L s-1. One of the larger macropores had water with high concentration of alkalinity, Ca and Si in dry periods, whereas during wet periods dissolved organic carbon was elevated and pH was depressed. This is a indication of different source areas depending on the rainfall rate. As the alkalinity/chloride ratios were lower in the macropores than in the peat water, acid groundwater seemed to find a shortcut directly into the lake. ¿ American Geophysical Union 1996