Here is the equipment for taking a water-quality sample from a large
river. It is pretty much the same equipment used for taking a river discharge
measurement,
except there's what looks like a big, white fish at
the end of the cable. Just as the speed of the water in a river varies
across the width of the river, things that affect the water quality
vary, too. For instance, a tributary coming in on the left bank of
the river might be draining a
chicken farm.
The water flowing along the left back of the river downstream of where the
tributary comes in will have different characteristics than water
flowing along the right side of the river, at least until it all gets
mixed together further downstream. That is why it is important
to sample at different spots across the width of the river.
The crane is on wheels, so the operator can easily take samples at different points along the bridge. A bottle is put into the head of the "fish" and then the sampler is lowered into the river. The sampler has to be heavy enough so it will sink and not be swept away by the flow of the river, which is why it weighs about 80 pounds.
Notice there are four heavy weights at the bottom of the crane to
counterbalance the weight of the sample. Yes, that "fish sampler" is made of
heavy steel -- if you want to prove your strength you could crank it
back up by hand. Luckily there is a battery-powered motor to reel the
sampler back up from the water. Don't tell anybody, but it's
happened that an operator has let the electric crank run a split second too
long on the way up, and the sampler flipped up and over the end of the
reel -- very embarrassing for an experienced technician.
Measuring water
Human's influence
Water use
Water Science home page
U.S. Department of the Interior | U.S. Geological Survey
URL: http://ga.water.usgs.gov/edu/bridgesampler.html
Page Contact Information: Howard Perlman
Page Last Modified: Friday, 07-Nov-2008 15:42:26 EST
