Why is there a difference reference for flood stages, and any other kinds of stages, I guess, between rivers and lakes?
Here is part of a flood alert from the National Weather Service.
The lake stage is given in elevation which makes sense. But the river stage is given in depth.
I assume these are abitrary points along the river, like a bridge pier near a town. Do they do it this way on rivers with barge and other commercial traffic? What happens when they dredge the channel, do they have to remark the bridge pier?
Like many of the “lakes” in the US, Lake Worth is actually a man-made reservoir, created by a dam built in 1916. Dams, being the major construction projects they are, often are designed and built using elevations (MSL) as references. Also, reservoirs tend to cover large areas and are designed with specific storage allocations, two more good reasons for adopting the elevation convention. Storage can be readily calculated through the use of topographical maps, which generally show contours marked with elevations. In other words, elevations were in use from the get-go at Lake Worth, and have remained as the stage reference to this day. Most reservoirs use the same convention.
Rivers and streams, on the other hand, are typically longer and more subject to localized flooding as a result of local weather. Also, river and stream stages have been tracked a lot longer than that of most reservoirs. Local observers, in need of some way to quantify the flood danger to their neighbors, and having no idea what the local elevations were, used what came to hand, such as, as you surmised, marks on a bridge pier or a fence post or a tree. The markings were assigned numbers quite arbitrarily most of the time, and eventually the readings became meaningful to the local residents. Of course, any of these gauges could easily be changed to reflect elevations relative to mean sea level, but why bother? There is such a thing as giving folks more information than they need, and no worthwhile purpose would be served by dragging a lot of folks through a new learning curve. In fact, I believe when USGS updates its equipment at a particular river or stream gauging station, it purposely retains the pre-existing convention.
Since no one else has answered your question in the last three hours, guess I’ll give it a stab.
You have the right idea about stream guages. They are arbitrary located guages established for a long enough time to predict at what point the stream will overflow its channel.
Dredging is not going to affect the level of the water. The water level will remain the same even if the bottom is excavated.
I thought it was probably arbitrary. But to follow up a little. TBone2 said: <<The markings were assigned numbers quite arbitrarily most of the time, and eventually the readings became meaningful to the local residents.>>
So, is that suppose to mean that when they said ‘flood stage is at 20 feet’, that it isn’t referencing the depth? I realize the depth has changed over the years, which makes it even more arbitrary, but surely it meant something at some time.
And bare said: <<Dredging is not going to affect the level of the water. The water level will remain the same even if the bottom is excavated>>
I realize the water level doesn’t actually change because of dredging, but if the number meant the depth, then the number would need to change. In other words, the flood stage would stay at the same big red mark, but if you took 5 feet of mud out of the bottom, the depth would now be 25 feet.
That wasn’t very clear, but what I’m getting at is that if the number are logically meaningless, then why not change them to something that is? We don’t live in a time when people actually go out to the bridge to see if the river is going to overflow soon. And even if they did, what the numbers are doesn’t make as much difference as where the big red mark is that says “flood stage!”
Thanks for the answers, it’s about what I thought.
Jim
No, Jim, the numbers never had anything to do with depth, but everything to do with elevation. I know that this statement tends to contradict what I said earlier, but it’s true nevertheless. Think of it as “elevation by local reference.” That is, “The base of that fencepost is 5. Anything above that is more than 5.” It’s all about reference anyhow; USGS likes to use “elevations,” which themselves are based on “mean sea level” (MSL), which itself fluctuates from minute to minute and from location to location.
Again, Jim, it’s not the depth that counts, but the elevation. People don’t get flooded out because the water’s too deep, but because the water’s too high.
Jim, the numbers ARE meaningful, locally speaking! In many regions of the US, true MSL elevations would be in the four-digit range, in some places five digits, plus decimals. Why confuse folks?
What would be more useful to you? “The Trinity is at 10.7 feet; flood stage is 13 feet; looking for a crest at 12.8 feet.” Or, “The Trinity is at elevation 514.67 and flood stage is 516.98. looking for a crest at 516.77.” And you should be aware that the day when people actually walked down to the bridge and looked at the water level relative to the markings is not far in the past.
River flood stage is not measured to the depth of the river channel. It is measured to the depth above the “normal” water mark on the banks. (This is usually taken in the dry summer when the water is at its lowest.) A flood of 20 feet indicates that the water is 20 feet higher along the shore than it would normally be. In a city where the river travels between high banks, that might mean only that the street approaches to a bridge or a few river-front stores will be affected. In a city where they need a levee to keep the water out every Spring, 20 feet may indicate the flooding of the entire town.
Nearly all streams and rivers of any size in the U.S. have automatic guaging stations that record and transmit water levels to the U.S.G.S. or other concerned entity. You have probably seen them yourself. Often they look like small towers along the side of the water body, sometimes they look like a large diameter pipe or culvert.