The USGS Water Science School
Checking the water quality of the Nation's streams, rivers, and lakes is one of the main responsibilities of the U.S. Geological Survey. Some measurements, such as water temperature and pH, are taken right in the field. But chemical analyses of the water must be done in a laboratory.
The hydrologist (water scientist) in this picture is taking a water-quality sample from a small stream. There is a bottle inside the white container at the end of the pole. You might think, "Why not just dunk the bottle in the water?" That wouldn't work because the quality of the water might be different at the stream's surface than it is near the stream bed, where heavier particles like dirt are flowing along. The velocity of the moving water varies vertically (as well as horizontally) in a stream, and so do the physical and chemical constituents that determine water quality.
A proper sample can only be taken by getting an equal amount of water from all along a vertical line up from the surface to the stream bed. On this sampler there is a nozzle at the end that goes into the bottle and lets in a consistent amount of water while the sampler is submerged. The hydrologist has to move the sampler up and down at a steady rate until the bottle is filled, while at the same time being sure not to smash the nozzle into the mud on the stream bed!