So, Why Didn’t the Forests Prevent the Recent Flooding?
Commentary by Larry Hamilton, Tree Warden,
May 5, 2011, page 2.....

The week of April 25 saw serious flooding in Vermont, especially in the Lamoille, Mississquoi and Winooski Rivers. Lake Champlain was over flood stage for several days, with lakeshore erosion from buffeting waves created by strong winds. Charlotte’s Lewis Creek and La Platte River reclaimed much of their traditional floodplain and carried much sediment. We were told that the high rainfall was causing headwaters to produce large water discharges that fed into these swollen watercourses. With so much of Vermont in forest cover, why didn’t trees prevent this flooding? We have often heard in past times that “forests can control floods.”
It is important to recognize that water storage on watershed land is not in the trees, but in the soil. Thus the depth of the soil is a key factor. Paved-over soil (no soil) in streets, highways, driveways or roof areas result in immediate runoff, starting water on its “flashy” way down to streams. Soils that are shallow over bedrock are a little better, but storage is very limited (steep, shallow mountain slopes, rocky escarpments and cliffs). Chittenden and Addison Counties have large areas of poorly-drained soils, where impediments to water infiltration may occur at 10-15 inches in depth – not a lot of storage capacity for prolonged or heavy rain. Deep, well-drained soils offer the best storage, producing the least runoff and greatest delay in runoff.
The other big factor is the antecedent water already in storage before the storm event in question. If there has been a period of heavy rain or snowmelt prior to a big storm, soil storage capacity may already be full or near full.
Here is where trees come into play in flood reduction and peak flow delay. When in leaf, especially in daylight and when it is above freezing, trees act as pumps. They both evaporate water and transpire water back into the atmosphere. Transpiration removes water from soil storage, and because tree roots are deep and plentiful, trees more than any other vegetative cover leave the soil storage capacity in the best status to receive new rainfall. Hence forests are the best land cover for watersheds and have the greatest positive impact on reducing peak flows and delaying peak flows. But if trees are dormant, or if rainfall exceeds the storage capacity of the soil, there will be both surface and sub-surface runoff. This discharge may exceed the stream or river capacity to handle the water, and a flood results.
Trees come into their own in another way – by reducing erosion. Surface erosion is less under forests than any other land cover. Moreover, tree roots anchor the soil against shallow landslips that occur when soils are super-saturated. Keeping sediment out of our streams and lakes is a very significant service.
There is, however, one topographic situation and special vegetative cover that beats trees, and that is wetlands. As a forester, I hate to admit it, but these are Mother Nature’s most valuable flood reduction gift. I hope that you have noticed during this flood situation how these wetland ecosystems have increased their impoundment depth and expanded laterally due to low topographic relief. (See the Bean Road wetland photo below.) Charlotte is blessed with an abundance of wetlands, as a quick look at the wetlands map in the Town Plan will show. These provide a myriad of values and services, perhaps the greatest of which is ameliorating floods and recharging groundwater. Wetlands merit protection from drainage, pollution damage and development, and establishment of wide peripheral buffer zones. The Town Plan and Land Use Regulations currently call for some protection of wetlands. But let us also not forget the importance of conserving as much of our upland and wet forests as watershed cover, wildlife habitat, scenic amenity and producers of sustainable products.
Prolonged and /or high intensity rain is apt to cause flooding, as we have noted recently. Neither dams nor levees, nor even forests and wetlands, can prevent floods, but the land cover is important in flood reduction. Forest and wetland conservation is cheap insurance for amelioration.