Activity in the 20th century decreased from the 19th century. Numerous small debris avalanches fell from Sherman Peak and descended the Boulder Glacier; a large one occurred on July 27, 2007. In early March 1975, a dramatic increase in fumarolic activity and snow melt in the Sherman Crater area raised concern that an eruption might be imminent. Heat flow increased more than tenfold. Additional monitoring equipment was installed and several geophysical surveys were conducted to try to detect the movement of magma. The increased thermal activity prompted public officials and Puget Power to temporarily close public access to the popular Baker Lake recreation area and to lower the reservoir's water level by . If those actions had not been taken, significant avalanches of debris from the Sherman Crater area could have swept directly into the reservoir, triggering a disastrous wave that could have caused human fatalities and damage to the reservoir. Other than the increased heat flow, few anomalies were recorded during the geophysical surveys, nor were any other precursory activities observed that would indicate that magma was moving up into the volcano. Several small lahars formed from material ejected onto the surrounding glaciers and acidic water was discharged into Baker Lake for many months.