Ion Export from a Small British Columbia Watershed After Forest Harvesting

A small coastal British Columbia lake was monitored before and after approximately 11% of the watershed was logged. There was an increase in the concentrations of most of the ions measured in the lake water after logging. Peak concentrations occurred two to three years after logging and most ions returned to background concentrations in fi ve to eight years. Two methods were used to estimate the increased export of ions resulting from the forest harvesting. The fi rst method used a water chemistry / hydrology watershed model, and the second used the changes in lake concentrations to calculate watershed input. The changes in ion concentrations and export varied with model and chemistry parameter with a change from a 1.3- to 4.2-fold increase for major cations and anions like calcium, magnesium, sodium, and chloride, to a 5- to 35-fold increase in ion export for nitrate and ammonium. The export of nitrogen from the watershed has implications for both water quality and future forest productivity. The measurement of lake ion increase is proposed as a general means of evaluating longer term change or disturbance in a watershed.