If you need to know where a Maine river is — how big they are, whether the fishing is any good — you just jump online and look it up, right?
There are no real mysteries about where our rivers are. This state and continent have been pretty thoroughly explored, and our cartography abilities have gotten pretty good over the years. Right?
Actually, the traditional methods for determining the widths of rivers, in particular, can still be wildly inaccurate, according to NASA.
And those faulty assumptions have likely caused scientists to underestimate how much rivers contribute to greenhouse gas emissions.
(Yes, that NASA. You’ll see how space comes into things soon enough.)
“Traditionally, hydrologists have estimated the width of rivers by examining topographic maps and backing out calculations from the amount of river discharge measured at certain points along the watershed,” the space agency reports, in part. “While this approach offers a ballpark estimate, other factors — such as the cohesive strength of the sediment in a river’s banks or the presence of dams — can throw such estimates off significantly.”
And I guess dragging a tape measure from one side to the other just takes too long.
So University of North Carolina hydrologists George Allen and Tamlin Pavelsky developed an algorithm to brighten the water found on satellite images and darken everything else, according to tech publication Wired.
They applied the algorithm to nearly 1,800 Landsat satellite images (there’s your space connection) to create a database of North American river widths.
But even then, the project wasn’t as easy as plugging an equation into the machine and watching a map of rivers come out the other end.
Allen and Pavelsky’s team had to painstakingly go through the images by hand and take out “water” that wasn’t part of a river — thousands of ponds, lakes and even swamps cluttered up the maps. They also had to pluck out little man-made bridges from all over the map, which unnaturally disconnected sections of rivers.
Additionally, they had to cross-reference the dates of the satellite images with the seasonal flows of the rivers to make sure they were getting images of each river’s statistical average flow, and also find images not heavily obscured by cloud cover or snow (which, as a form of water, threatened to throw off the algorithm).
And that’s not all.
“Because Landsat’s 15-meter resolution makes it impossible to measure rivers below a certain size, Allen looked at the width distribution of larger rivers, which fell almost perfectly on a power-law distribution, and extrapolated that relationship to the smaller rivers and streams,” Wired reported.
So pulling this map of North America’s flowing waters took many steps and was very time consuming.
Still, why go through all this trouble?
According to Wired, the project is expected to help gauge flood risk and climate change across the continent — and it ultimately determined that much more of North America is covered by rivers than ever thought, due to the underestimation of river widths created by previous measurement methods.
Studying river widths to better understand flood risks is fairly self explanatory.
On the climate change side, Allen said, the news is that rivers actually contribute to greenhouse gas emissions.
“Most people haven’t heard of this, that rivers do emit quite a bit of [carbon dioxide] naturally,” Allen told Wired. “It’s sort of an emerging field.”
“Microbes in groundwater produce CO2 as they breathe and supersaturate the water with it. That water then flows into streams and the carbon dioxide is released into the air. Scientists are still working out how much of that CO2 escapes into the atmosphere, but the rate of exchange between the water and the atmosphere will depend partly on the amount of surface area exposed.”
And thanks to the latest river-mapping effort, scientists now know there’s much more river surface area in North America than previously thought.