River Levels FAQ (v0.99)

Questions

• How did you do this?
• Can you add more rivers?
• What is the conversion factor between river stage & CFS?
• Can you show these plots in CFS?

For several years now, the National Weather Service has been providing text files containing weather-related information on the internet, available by ftp, gopher, and now the WWW. This page uses this part of this information to make its plots.

Each morning, a perl script on this machine ftp's over to the Oregon Climate Service's machine, where those files are held, and grabs the files it needs. It then rips the files apart, parses out the information it needs, and splits them up into seperate files for each location. Then it goes through each location and runs the data through gnuplot to make the plots you see. The table you start out with is generated when you load it by looking at the data files & taking the latest river level. When you click on a link in the table you are actually running another script which goes and grabs the relevant plots for that river.

Can you add more rivers?

Yes and no. This particular page is completely dependent on which rivers the National Weather Service decides to send out over the internet in its text files. Right now, those are pretty much only NW Oregon rivers. There are other sources of information, such as the US Army Corps of Engineers, or USGS, but they already have web sites, and you are welcome to wander over there. I see no need to duplicate their work. If the NWS adds more rivers to their files, then I will update my scripts and plots will start to appear...

What is the conversion factor between river stage and CFS?

There is no direct conversion factor between an arbitrary river flow (CFS, standing for cubic feet/second) and a river's stage (height, measured in feet). They are different measurements. River flows are a measure of the volume of water that passes by a point, and require one to obtain 3 pieces of information for a specific location: How high the river is, how fast the river is moving, and the shape of the river bottom. To be more exact...

Look at the picture above. The upper black curvy line represents the shape of the river bottom at the spot where the gaging station measures the river's stage or height, (h). The blue mass is the amount of water that passes through that section during time, (t). You can think it as being something like blue play-dough being squirted through a hole shaped like the river bottom where the gage station is... Anyway, the total volume of water that passes through this section is equal to the distance, (d) times the cross-sectional area (A). To get a flow, which is volume/unit time, you simply divide this by how long it takes to fill up that volume. This is equivalent to calculating the flow as being the cross-sectional area times the velocity of the water. But wait a minute -- where does the river stage come in?

The cross-sectional area depends on two things -- the shape of the bottom and how high the water is. If you look at the exaggerated picture below, you should notice that the same change in river height does not necessarily give you the same change in cross sectional area...

If this particular river was really low, a change of one unit in height would yield a small change in area. But when its higher the same change would give you a much bigger change in the cross sectional area.

This says that you can't have a constant conversion factor to get cross sectional area from the height. But, if you send a graduate student out in a boat to measure the shape of the bottom, it is still possible to calculate what the change in cross-sectional area, and therefore the flow, is depending on the river stage. This is done with a Rating Table. These take into account the shape of the bottom of the river, and give a table of conversion factors, depending on how high the river is.

So the short answer is, yes, there is a conversion factor, but it only works for a specific river level.

Can you show these plots in CFS?

In theory, yes, however its best that the professionals do this... As anybody who's been close to an Oregon river this winter can tell you, the shape of the river bottom changes from year to year, sometimes even from month to month... This means that the rating tables change as well, and I would have to be constantly checking for updates. The USGS list river flows on their site, and also deal with updating the rating tables -- They have the proper tools, best let them do the work...

dmason@zebu.uoregon.edu