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Comment by loeg | Hacker News Reader

> logged 100 years ago which impacts the hydrology of the basin in a negative way for the purpose collecting drinking water.

Can you elaborate on the problems?

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Trees effectively behave as a buffer for the water cycle. Under conditions of high rainfall, they absorb a lot of water from the ground. Under conditions of low rainfall, they must nonetheless continue to release moisture via evapotranspiration, which promotes the production of rain in dry conditions. This mean that forested locations are more resistant to both floods and droughts.

Tree roots also greatly reduce erosion, which means that rainfall is more likely to end up in a few well-established waterways and less likely to be spread across numerous tiny rivulets; it also means that waterways are less likely to change shape over time. Large, stable waterways are much easier to collect water from (e.g., via dammed reservoirs) than small or unstable waterways.

However, I'm not sure how much of a factor erosion is here. I suspect it will only directly impact small communities that rely on minor waterways for drinking water. (Mind you, when small communities connect up to city water because their existing water supply becomes unreliable, that can affect the city's water supply.)

Sure, but this region is still heavily forested -- just with second-growth forest (trees ~100 years old and not older). Is there a significant difference between old growth and second growth for this purpose?

A forest with larger trees and more extensive root systems will have a stronger effect than a forest with smaller trees with less extensive root systems.

Different tree species can also be more or less effective at functioning as a buffer. "Thirsty" trees typically do a better job of taking up water when it's wet and continuing to release water when it's dry. (Unfortunately, many new tree plantings favor more drought-resistant trees because they are easier to grow in clearcut fields, which are drier than forests.)

So, because I don't know much about this region specifically, these are my two questions:

Has there been a change in tree biomass in the region?

Has there been a change in the tree species makeup of the region?

From the recreational perspective (summer hiking / back-country skiing) - the forests are night and day difference. Having a map of what has been logged and what has not often is the difference between forests that are easy to travel through and forests that are harder. A forest that was clear cut will have trees that are much denser and tightly packed together but tend to be smaller in diameter. These are very hard to travel through (hiking or skiing) compared with the old growth forests.

I don't know a ton about forest ecology but my sense is that trees that do the best are a function of what's already there and that it takes much longer than 100 years for the pre-clear cut conditions to return.

The Cedar River Watershed is not open to recreational use (it's a protected pristine watershed that supplies drinking water to Seattle and surrounding suburbs). The difference for that use is not relevant here.

They are most likely referring to the Cedar River Watershed, which supplies 70% of Seattle's water. While the city spent the 20th century buying up all the land so that it is now a protected wilderness area, plenty of logging happened during that time and less than a fifth of the old-growth forest remains. You can read all about it here:

https://www.seattle.gov/utilities/protecting-our-environment...

Specifically, here is the forest management plan, which goes into great detail about the current conditions, their effects on the water cycle, and the long term objectives:

https://www.seattle.gov/documents/Departments/SPU/Environmen...

I spent a few years in my 20s volunteering for ecological restoration projects in the watershed. We dug up old logging roads, removed invasive species (Japanese knotweed, ugh!), deconstructed landscaping left over from abandoned small towns, did erosion control along creeks in logged areas, restored riverside habitats, and planted lots of native trees and shrubs. I am not in touch with the organization anymore, but I'll always feel some pride in the work we did and a sense of connection to the place.

Yes, I assumed they were talking about the Cedar River Watershed (Chester Morse lake basin).

> Specifically, here is the forest management plan, which goes into great detail about the current conditions, their effects on the water cycle, and the long term objectives:

This is a 130 page document. The first few pages mentioning old-growth forest are mostly discussing habitat for fauna. Is there a more specific part of the document discussing hydrological impact?

> I spent a few years in my 20s volunteering for ecological restoration projects in the watershed. We dug up old logging roads, removed invasive species (Japanese knotweed, ugh!), deconstructed landscaping left over from abandoned small towns, did erosion control along creeks in logged areas, restored riverside habitats, and planted lots of native trees and shrubs. I am not in touch with the organization anymore, but I'll always feel some pride in the work we did and a sense of connection to the place.

Very cool! My only connection to this is that my mom worked for SPU in drinking water.

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