As “flattening the curve” became so well-known and commonly referred to during the Corona pandemic, I thought it made a good segue into watershed management.
This probably looks familiar. It is a graphic fromthe CDC about how a sharpened peak exceeds our hospital capacity, but a flattened curve keeps us within what we can manage.
Here is a very similar looking graph, but this refers to the flow of water down a stream or river after a rain. When we build homes and roads and even lawns, we actually increase the rain runoff. Just as you have seen it sheet on roads, homes and driveways are no exceptions. So in a developed setting, we have a sharper curve with faster and more runoff than in an rural setting. Note that a sharp curve is more likely to go beyond the capacity of the river and flood the surrounding area, while a flattened curve is less likely to flood the banks. Similarly, if you have been out in a desert during a rainstorm, the water flows fast without vegetation and you need to get a move on quick.
Here is a real life example. This is the Schuykill river’s discharge in Philadelphia, chosen because it is far from its source and in a sprawling urban area. See how sharp that peak is. The flow increased by over 25000 cubic feet per second and caused the water level of the river to rise by three feet, which was not a flooding problem for this area, but also not a small amount. Go to https://waterdata.usgs.gov/nwis/rt and you can probably find a site near where you live (if you are in the US) and find all sorts of data (click on your state first). It is pretty cool that the government has equipment to get this data all over the country, easily available for anyone.
Apologies if I misinterpret this picture I took off googlemaps. Here we have farmland along the Red River in the northern Midwest. Sometimes it floods a lot up there and you may wonder “why would it flood in an area without a lot of development? Rural areas have flattened curves.” Well, the river’s floodplain used to be expansive, and a lot of it was a very wet prairie. Well, we Americans wanted to farm this wet prairie, so we had to drain the land somewhat and dug a series of ditches to shove the water off to the river. So by increasing the speed of water flowing into the river, we make that curve peak sharper. This may not be as bad as if it were all roads and houses, but it does help increase flooding. There is a chance that since I have never visited this particular site, I could be mistaken, but what I described is common throughout the country, and this was a very good visual example.
If you have ever walked through a forest during a rainfall, you may have noticed you either never got wet, or had to rain awhile before you did. So, again, cutting down the trees will increase the rain that gets to the ground and sheets into rivers, increasing peak flow. So if you have land you want to develop along a river, you may discover regulations that say “do not build along a stream or riverbank” or “do not fill in those wetlands” it is actually saying “help keep the curve flat” or “please don’t flood downstream and kill your neighbor.” It may also simply be a dangerous place to build, but may also have been fine without the increase in river flow pre-development. Obviously a natural system can flood, but they flood far less often. There is almost a cat and mouse game in the US where developers put homes in flood prone. Though not illegal-to-build-on areas, over time they turn into massive flood hazards and the government then either buys them out or funds their flood insurance for decades, spending millions needlessly. This picture is before and after a home buyout in Houston, borrowed from https://www.hcfcd.org/Hurricane-Harvey/Home-Buyout-Program
Obviously there are also ecological reasons to protect rivers in these ways. A shaded waterway is cooler, which means it can carry more oxygen, allowing fish to survive. If you like that fish exist, or like to fish fish, then you probably consider this to be a good thing. Here is more reading from Utah State University https://extension.usu.edu/waterquality/files-ou/whats-in-your-water/do/NR_WQ_2005-16dissolvedoxygen.pdf
Other critters live in the water as well. A dragonfly larvae may actually take years to mature in the water. The oxygen from cooler temperatures help it survive. So does an ecosystem filled with other critters for it to eat. For instance, leaf litter falls into the stream from the shaded banks and many species of insect larvae will then eat the litter, which in turn may become a source of food for the dragonfly larvae. Here is a pic of a dragonfly larvae underwater that I stole from the Missouri Department of Conservation. Check them out: https://nature.mdc.mo.gov/discover-nature/field-guide/dragonfly-larvae
Here is a picture of some happy cows taking a bath. This looks nice, but is actually a problem for rivers all over the world. The massive amounts of cow manure actually overloads the waterways with nutrients, causing significant ecological problems, even far out into the ocean. Ways to help mitigate the flow of manure into streams include 1) keep cows out and 2) have a buffer of trees and other vegetation along the stream (often recommended to be at least 50 feet wide). This filters out a lot of the manure (also fertilizers, pesticides, etc.) while also helping to prevent flooding and providing shade and leaf litter for the stream ecosystem. I yoinked this pic from www.potomacriverkeepernetwork.org where they work to protect rivers.
Now keep in mind, an individual developer likely would not know all this, and if they did, they probably think their spot is ok, since it is legal to build there. Either way, it is never one development that causes these problems, it is the aggregate of many. The same goes for the farmer like Jesús here or the people living up by the Red River, who are also earning a living. These problems are the result of a society and require collective problem-solving techniques (often government buy-outs, regulations, and good neighbors being neighborly). That said, there are developers who are told an area is off-limits due to flood data but then fight with the government to build in a spot by using their own data. They often win and… well, you can imagine the results.
Hopefully you now better understand flattening the curve, riparian regulations, and river ecology and are now ready to appreciate a happy cow. There is certainly a lot more detail I could have thrown in here as well as nuance and site-specific exceptions, but I was aiming for short and digestible.
