Redwood Reader

9. Re: How Big Is A 100 Year Flood? Many years of working in watershed restoration have shown that floods are not the result of large storms but destruction of water holding capacity in the watersheds, which slows the passage of water through the system and allowing the river to carry the water off at a rate that does not jump the banks or scour the banks creating the “river bar”. Our civilization has impacted this regime in many ways, and it is our practices that create many of these flood events. The rate and manner of development in the watershed is actually more important than the amount of rain for the simple reason that removal of vegetation destroys the water holding and delaying capacity of the forest and soil, which is directly related to glomalin produced by the vegetation and stored in the ground. Engineers and technicians can only work within the accepted frameworks, and so must wait for science to prove itself not only as true but as do-able, technically and fiscally. The multiple floods mentioned say nothing about changes throughout the watershed preceding these events. It is likely the ability of the landscape to intercept the rain was repeatedly impaired, making comparisons between events not so helpful.
Here in Humboldt County, without large metro areas, we had some horrific events like the flood of 1964. Much debate has swirled over the causes, and the Forest Practice rules of 1972 were introduced. There was snowpack and a warm period, heavy rain, and one other factor to consider: abnormally high tides blocking the river mouths from emptying into the sea. Instead, the water backed up in the rivers to an amazing depth and scouring the banks for many feet from the channel and for miles up the rivers and tributaries, which is why it is so hard to reestablish shaded areas along the rivers for summer.
Massive caterpillar logging since WWII had removed canopy, radically altered drainage on the ground, choked the river and streams with sediment and reducing their capacity. Runoff was created on a massive scale. Altered and overwhelmed drainages carved into the ground and carried huge portions of the landscape with it. The soil is a marine sediment with barely a rock in it, liquefied in hundreds of areas causing slides, debris torrents and mass wasting. Huge amounts of sediment poured in from devastated tributaries, filling ancient sturgeon holes forty feet deep in the main rivers, and altering our Chinook, coho and lamprey runs. The study site is a tributary of the Mattole River and just over the hill from Humboldt Redwoods State Park. HRSP is an interesting aside. Save-the –Redwoods bought large groves of redwoods to protect into the future back in the twenties. The location in Bull Creek was in the lower river, and private property shared the watershed with them. Massive tractor logging went on in the upper watershed, above Cuneo Creek on Big Hill. When Big Hill failed, and the other upper Bull Creek and Panther Creek areas, the water took out hundreds of huge ancient trees like toothpicks. As a result, Save the Redwoods League purchased the Bull Creek watershed headwall to headwall for the park in order to stabilize the landscape and prevent further harm, to this limited resource. HRSP General Plan currently includes Bull Creek Restoration project, including pool building and tree planting which may well be a model for other impacted watersheds.
Glomalin accumulates over years making soil porous and decreasing water repellency. It is destroyed by sunlight, running water and ambient air. When it is destroyed, it reverts back to CO2. For this reason I believe the amount of CO2 contributed by emissions is overestimated, and the amount of CO2 pumped into the atmosphere by farming, logging, roads development and construction needs to be quantified., all of which also alter the grounds ability to absorb water. Remember, glomalin was only discovered in 1996 and is only now being regarded in non-crop situations. Even for crops, though, glomalin is changing practices to use no-till methods, storing carbon while decreasing irrigation and runoff. I think they will find more glomalin in undisturbed forest than anywhere else.
Glomalin also gives us hope for the future, as it is created at a much higher rate in enriched CO2 atmospheres. That is to say, the more CO2 in the air the faster our watersheds will be able to heal themselves, increasing their precipitation retarding properties above ground and below increasing the storage capacity. Glomalin is created even faster when temperature is increased. This is a real management opportunity for planners. Break out the maps and see where you have altered the soil structure through roads, farms, logging, development. As in all watershed issues, you need more trees, and in this way you can actually shrink the size of a hundred year flood.