Trees interrupt the rain which falls in their vicinity, capturing it in on their leaves and returning it to the atmosphere by evaporation. They also soak up water from the ground and send it back into the atmosphere from their leaves by transpiration. Removing mountainside trees increases the amount of water running off the slope by two to five times or more depending on conditions, causing a wide variety of slopeside and downhill drainage effects.
Removal of trees on a mountainside will have an immediate, obvious effect on the mountainside soil. These include water flooding, sheeting, channeling and cutting gullies. Increased water flow together with a dramatic shift in available light will also alter the species components of vegetation in the created opening. A study conducted by the Soil Sciences Department of the University of Saskatchewan indicated that surface run-off from clear-cut slopes increased five-fold during snowmelt over the run-off from forested hillsides. These ground erosion effects will accordingly be significantly escalated in the spring melt season in areas that experience winter snowfall.
Significantly increased volumes of water flowing downhill over the mountainside surface will raise stream water levels and may cause downstream flooding. If the flooding exceeds infrastructure design parameters, it may damage culverts and bridges as well as cut away stream banks and potentially cause streams to jump banks forming oxbows and yazoos. Soil carried downhill by the water will cause stream sedimentation and may clog storm drains and cause surface deposits of sand and silt material.
Water Table Impacts
Well and water table drainage effects of mountainside tree removal will vary dependent on the topography. In areas where the water table is refilled from stream flow, or where a water body such as a pond or lake will retain the added surface water run-off from tree cutting, well levels and water tables may rise after up-slope tree removal. However, in many environments, increased quantities of rapid surface-water run-off do not serve to replenish ground water systems.
Although tree leaves intercept rainwater between the sky and ground, they also increase the rate at which water which does reach the ground infiltrates the soil. Mature tree roots help maintain an open, permeable soil structure from the surface to depths of several dozen feet. Grasses and early successional species which grow where mountainside trees are removed have denser, shallower root structures which have lower water infiltration rates, so less water will sink into the soil to replenish groundwater reserves.