- Persistent levels of high turbidity in the outflow from the
Prineville Reservoir led to development of relationships between
watershed soils, land use, and resource management and water
quality in the Upper Crooked River, Central Oregon.
Seven stations, strategically located for measurement of runoff
volume and water sample collection, were established late in 1972.
Six stations were on tributaries to the Crooked River; the seventh
was on the main stream above the Prineville Reservoir.
During runoff events, suspended sediment samples were
obtained on each tributary above the confluence with the Crooked
River. Greater stream sediment loads were associated with tributary
watersheds under shrub-grass-juniper cover that have steeply
rolling, dissected terrain with common rock outcrops. Those
watersheds also have dominantly medium-textured, moderately deep
soils. Steeper gradients are associated with these watersheds.
Streams contributing high sediment loads were not necessarily
those found to cause the long-term turbidity. This phenomenon was
usually associated with those watersheds having a preponderance of
soft, tuffaceous sedimentary rock.
The results indicate major runoff events carry the greatest
sediment loads to the reservoir and cause considerable turbidity.
However, some turbidity-causing material is transported during
smaller events as well. Freshets on watersheds with erodible soils
cause turbidity during the convective storm season. Turbidity
values in streams decreased between storm and runoff events,
especially on forested watersheds. Sensitive watersheds (Camp
Creek, TomVawn and Eagle Creek), with easily eroded soils,and
specific reaches of the main channel, supply disproportionate
amounts of material, causing long-term turbidity.
X-ray diffraction analyses of suspended sediments, and the
clay fraction of soil samples showed a predominance of smectites.
Significant amounts of amorphous material were associated with the
smectites in samples capable of creating long-term turbidity in the
Field reconnissance of watershed land use and management
indicated situations where domestic animal grazing, timber harvest
and associated road building, and fire prevention practices which may unduly contribute to erosion and turbidity problems. Primary consideration
of soil-hydrology relationships in land use planning and
management is needed to reduce the severe erosion observed on
certain tributaries. Particular attention should be directed toward
minimizing erosion on soil with characteristics associated with long-term
Reservoir management and control should include recreational
boating regulations to reduce shoreline turbulence. Limitations on
boat draft, engine size, and total use will aid in reducing summertime
Recommended conservation and stabilization practices include:
1. Riparian fencing with controlled access for protection of
stream bank soils, channel, and vegetation from external
2. Retention of felled junipers, and continued felling, to
extend soil moisture for germination and growth of
3. Selective stream channel clearing to reduce debris-caused
4. More awareness of soil-hydrology interrelationships by
management agencies and private owners to better protect
against turbidity-causing practices; and
5. Range management designed to maintain and improve
grass cover and range condition overall.