by Steve Nelle, NRCS, Texas
Watersheds or Catchments?
About 2400 years ago, the great historian and scholar, Plato described the condition of the landscape in ancient Greece as well as a dramatic change he observed. The description below is a paraphrase of the English translation:
“In the primitive state of the country, the mountains and hills were covered with soil and there was an abundance of timber. The plains were full of rich earth, bearing an abundance of food for cattle. Moreover, the land reaped the benefit of the annual rainfall, having an abundant supply of water in all places; receiving the rainfall into herself and storing it up in the soil. The land let off the water into the hollows which it absorbed from the heights, providing everywhere abundant fountains and rivers. Such was the state of the country, which was cultivated by true husbandmen, who made husbandry their business, and had a soil the best in the world and abundance of water.
“In comparison of what then was there now remain only the bones of the wasted body. All the richer and softer parts of the soil have fallen away with the mere skeleton of the land being left. For the fact is that a single night of excessive rain now washes away the earth, and lays bare the rock. Now the land is losing the water, which flows off the bare earth into the sea.”
In the earlier account, Plato described what must have been a near perfectly functioning water cycle. The land was well vegetated and the soil was rich. When it rained, the land caught the water and stored it. Excess water was slowly released and emerged as springs and flowing streams. As result of these things working together, the land produced a great bounty of timber and grazing. He went on to describe those early land managers as “husbandmen”, who apparently practiced a high form of land stewardship. Those lands can be accurately described as water catchments, not watersheds. After some unknown interval of time, Plato wrote a disturbing account of what had happened to the land. Erosion had been rampant, indicating an extreme loss of vegetation. The loss of vegetation and soil was so severe that bare rock was being exposed. Rainfall was quickly transformed into runoff and very little water was retained on the land. We can assume that springs declined or dried up and streams ceased flowing continuously. We might be able to imagine the condition of the riparian areas. We can also assume that sedimentation was greatly accelerated and water quality declined. This land could accurately be described as a watershed, but not a water catchment.
The question we must ask ourselves is – are our lands functioning more as watersheds or water catchments? As we consider the land and the water cycle, we would do well to revise our thinking, our management and our terminology to emphasize catching water instead of shedding water.
A large rainfall event several years ago on the North Concho River Watershed produced rapid runoff and out-of-bank flow. The floodwater looked like rich chocolate milk flowing down the river. A one gallon sample of this runoff from the main channel was collected and filtered. It contained 12.8 grams of sediment. That may not sound like a lot, but this concentration is the equivalent of 4.6 tons of sediment per acre foot of runoff. This one storm event contributed 7400 acre feet of muddy water in O. C. Fisher reservoir, north of San Angelo, along with an estimated 34,000 tons of soil.
Riparian researchers have shown that the majority of sediment in rivers is not coming from upland watersheds, but instead from eroding and unstable banks, channels and floodplains. They estimate that 80% of river and creek sediment is coming from the riparian area itself. Stabilizing channels, banks and floodplains can do a great deal toward improving water quality and reducing the excess movement of sediment.
A riparian area that is functioning properly will support a heavy stand of densely rooted upright vegetation. Riparian plant species have different rooting characteristics than upland plants. Root systems of riparian vegetation are denser and stronger than upland species. This dense, strong root mass is one of the critical factors in maintaining bank and channel stability. The power of rushing flood.water is immense and an equally immense network of roots to support and reinforce wet banks and riparian soils is needed.
A study of the root system of an important western riparian sedge (Nebraska sedge) revealed an amazing fact: One cubic foot of soil contained 21 miles of roots! The stability rating of this sedge community was equal to the stability of anchored rock. By contrast, a common upland grass of the same region (Nevada bluegrass) had about 2 miles of roots and a stability rating of only 30% as great as Nebraska sedge.
Another study of the Platte River riparian system showed that root biomass was over 4 times more than above ground biomass. Root biomass was about 21,700 pounds per acre consisting of grasses, sedges and forbs, while above ground production was about 4,700 pounds per acre.
A certain amount of erosion and deposition is normal and natural in river and creek bottom areas. However, when bank erosion becomes excessive, it is a sure sign that erosion and deposition are out of balance and the riparian vegetation is inadequate.
Three broad types of riparian vegetation help provide needed stability:
- Colonizer plants very quickly spread and put down a mat of new roots by stolons or rhizomes (knotgrass, spikerush, some sedges, water hyssop, water primrose).
- Stabilizer plants are usually taller upright plants with strong dense root masses (switchgrass, Emory sedge, sawgrass, eastern gammagrass, bushy bluestem).
- Riparian Woody plants with larger diameter roots function as “riparian rebar” (black willow, sycamore, button bush, little walnut, baccharis, indigobush amorpha).
“Riparian roots reinforce riverbanks, reducing the reckless rage of rampant runoff”