The acidification of lakes and streams as a result of acid precipitation has been documented in southern Sweden and Norway, the northeastern United States, and southern Ontario. Simple and complex models have been used to assess the impact of acidic deposition on lakes and streams. One model development (the Trickle-down model) places particular emphasis on the kinetics of chemical weathering in the watershed as the primary mechanism that produces cations (Ca(+2), Mg(+2), K(+), Na(+)) and neutralizes acid inputs. In the study the Trickle-down model was used to simulate lake response and to predict the percentage of lakes in the Upper Midwest of the United States at risk at various levels of acidic deposition.