You may have noticed that after a heavy rainstorm, creeks and rivers often turn the color of chocolate milk. That cloudy brown color is caused by sediments—weathered rock material ranging in size from tiny granules of mud to stones. As it courses along, water sweeps up sediments in the well-known process of erosion. Eventually, the sediments find a home, sometimes in a place where it isn’t wanted. And, it’s not just mud and sand that gets carried to water sources. Contaminants often catch a ride to waterways by clinging to sediments.
Soil Scientist David Lobb has investigated the origin of these sediments. A variety of human activities can negatively impact the make up of these sediments. Fertilizers used to bolster crop yields, sewage pollution from treatment facilities, and waste from livestock can leach an excess of nutrients. The nutrients, especially phosphorus, then enter large bodies of water. Lobb and his colleagues from the University of Manitoba and the University of Northern British Columbia use a technique called color fingerprinting.
The color of a particular sediment is key to identifying the specific origin of the erosion. It’s also cheap and quick. “In the most simple case, black sediment is from surface sources and light sediment is from subsurface,” says Lobb, “That’s an oversimplification of a very precise process backed up by statistical models.” The distinction between surface and subsurface sediment sources is important. Subsurface sediment, or subsoil, is usually pulled by the water from the sides or bottoms of streams. Surface sediment, or topsoil, is more than likely coming from farm fields, riparian areas, or forest floors.
Most of the sediment found in the South Tobacco Creek is from subsurface sources. It’s coming from the stream banks and the huge rock walls that borders the creek as it cuts through a 600-foot escarpment. “Most people assumed sedimentation is caused by erosion in farm fields,” says Lobb. “But one of the biggest culprits is the natural channel erosion that is constantly taking place.”
The color coding technique makes it simple to find the geographic origin of sediment. “What to do with these answers is not as simple,” say Lobb, “but precise color fingerprinting allows technology to open up to new directions. We are now looking at managing runoff from farm fields as being as important as managing erosion and sediment losses from farm fields. And, we are looking at managing runoff and erosion from the farm field scale to the watershed scale.”
Source: Soil Science Society of America
