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Topsoil, Flood plains, and Us

May 27, 2011

Montana is flooding right now, as is much of the country. This has immediate interest for Mr. B and I because we bought a flood plain. Out of 9 acres, at least half is considered to be in the 100-year flood plain by FEMA, and up to 90% is flood plain if you talk to the county flood plain administrator.

Our property is bordered by a creek with an overflow channel running through part of the property. It’s hard to see in this picture, but there is an undersized culvert in the creek. This is really why the overflow channel exists – the culvert is too small to let all the water through. If our property floods it will be because of this tiny culvert.

The area we live in is a long valley with a creek running through the middle. According to Missoula-natives, the area used to be three farms and was mostly pasture. The creek wandered where it wanted and the whole valley flooded almost every year. As the farms were converted into smaller parcels, the creek was progressively channelized and the banks built up with berms. Even if our property doesn’t flood to the extent it once did, the soil is valley bottom, flood plain soil.

What does this mean for us? Honestly, the way I’ve been thinking about it is just as a hurdle. We had to move our proposed house site because of the flood plain, and it makes it difficult to get a septic system and well onto the property. Until this weekend I had just been thinking about flood plains in the abstract. Flood plains are some of the most fertile agricultural lands in the world. We bought a tiny Nile Delta – that was good. Streams provide nutrient subsidies, adding nutrients when they wash over their banks.

Then we saw this:

I don’t know how to put arrows on pictures, so bear with me. See that really dark layer of soil on the top? That is topsoil, or the A horizon. It is 2 – 3 feet deep over our entire property. This is a depth on par (I think) with the Great Plains pre-agriculture. Our property has been grass for at least 50 years, if not more. I’m not sure it has ever been plowed or planted. This type of soil depth is unheard of around most of Missoula – most people have about half an inch. Black gold. Topsoil is the part of the soil that holds the nutrients and where plant roots and soil animals actively move and grow. According to the NRCS soil map for our property, our topsoil is Grantsdale loam, which they consider to be prime farm land if irrigated. Hooray!

Unfortunately, topsoil is bad for building. Our excavator winced when he saw how deep the topsoil layer is, because all topsoil has to be removed to build a foundation. This means we have to change our foundation from a slab on grade to a crawl space with stem walls if we don’t want to spend our entire budget on excavating services and gravel.

I feel like I’ve been given a gift, but a gift that is easy to destroy. Topsoil loss is primarily caused by agriculture, albeit industrial or poorly managed agriculture. The Dust Bowl is just one example, where soil left bare was eroded by wind and rain until it all blew away. Our property might have a creek that can never be unchannelized and invasive weeds, but the soil is pristine.

2 Comments leave one →
  1. Cornelis permalink
    May 30, 2011 3:33 am

    I am happy to keep informed about your interesting “wild-life”

    Many friendly regards, Cornelis

  2. Beth permalink
    May 31, 2011 9:07 am

    That dirt looks delicious!

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