Gray Merriam | May 24, 2017


Recently, a flood victim said "the walls I have built are not strong enough – I will make them stronger and bigger"

He summarized the widespread philosophy that pops up with every flood. Hold it back with walls of sandbags and keep it in its proper channel with berms. An engineering prescription for floods.

Stewardship of the watershed is a better, cheaper and longer-lasting prescription. Instead of 'here is the water—fight it', ask 'where does the water come from? How can the flood peak be prevented before it threatens us?'

Water that comes as rain and meltwater does not just run down our rivers. Rain is water that evaporated from the earth into the atmosphere. When it falls back to earth, it can be absorbed into soil or litter, or it can be evaporated again back to the atmosphere, or it can be contained in wetlands or lakes, or it can flow down a geological fault into the groundwater.

Prolonged or sudden, high volumes of rain or meltwater will quickly run off if it falls on waterproof slopes such as bedrock or pavement. Cover that area receiving the rain with materials that can absorb the rain, or let it soak in, or let it flow down faults and cracks into the groundwater, or evaporate it, or slow its flow downhill or store some in pockets in the land, such as wetlands, and the flow will be slower or the volume flowing will be less. The headwaters are the key.

Trees have great effects on the fate of water. For photosynthesis, trees use only about 1 percent of the water they take up; most of it is evaporated. Forest can remove up to 70 percent of the rainfall by evaporation and transpiration from the pores in their leaves. Because deciduous trees lose their leaves for several months and conifers do not, the conifers remove more water by evaporation and transpiration. In one experiment where 16 hectares (about 40 acres) of deciduous forest was replaced with conifers, about 24 million more litres of water was moved back to the atmosphere by evaporation and transpiration by the conifers.

Some trees can move 500 millimetres of rainfall back to the atmosphere in a year if the trees are supplied with lots of water. That is more than half the annual rainfall in our region.

A watershed with lots of conifers in the headwaters area of the watershed can reduce streamflow by 15 to 20 percent compared to watersheds with no forest.

Besides evapotranspiration sending some rainfall back into the atmosphere, forests, including their leaf litter and their dropped limbs and other large litter, interfere with the flow of surface runoff and allow more rain to penetrate into the soil.

Besides benefits to us in our watershed, forests are major conservers of biodiversity. Forests are important components of the beauty that we see in our landscapes.

Bags and berms is a 'bandaid' approach. But we could easily apply a longer-term and less costly solution to sudden flooding by planning our stewardship of land use in the headwaters of our watersheds.

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