The USGS Water Science School
Here's a couple of sturdy fellows enjoying a freezing day taking a water sample from a large river. (I can remember doing this when it was -5 degrees F, and that was in Georgia... I can imagine doing this in Montana). The equipment used here is for taking a water-quality sample from a large river. It is pretty much the same equipment used for making a stream-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, there might be an industry on the left bank of the river that discharges waste into the river. The water flowing along the left back of the river downstream of the industry might have a different quality than the 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 take samples easily 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. In very small streams the samples are taken without a crane.
These devices are called "depth-integrating samplers" because they allow the hydrologist to collect water at a constant rate throughout the complete vertical column of water (from surface to streambed). The sampler bottle has a small tube on the front which allows water to enter the bottle slowly and at a constant rate. This is important because the chemical composition of the water at the surface might be different than the that near the streambed.