Douglas B. Moog

Sediment-transporting flows in headwater streams

The equilibrium alluvial stream channel has a geometry that allows it to pass the water and sediment supplied from the watershed. At the same time, the equilibrium alluvial channel is built and maintained by the flows and sediment delivered to it. A prerequisite for understanding the creation of the equilibrium channel is an understanding of the sediment conveyance and competence of the flows the channel receives. This study describes the bed-load transport regime as it is linked to hydrology and geomorphology in 23 headwater gravel-bed streams in snowmelt-dominated parts of central and northern Idaho. At sites, drainage areas range from 1.29 to 381 km 2 , stream gradients range from 0.0042 to 0.0747, and median bed surface particle sizes range from 4 to 207 mm. Stream architecture includes riffle-pool, planar, and step-pool beds. The bed load is much finer than the surface and subsurface material, suggesting selective transport of the finer sizes. Nonetheless, the majority of the load is sand at all flow discharges. Progressively coarser sediment was collected as flow discharge increased, and painted rock experiments documented the transport of coarser particles at higher discharges. The supply of sediment to the streams appears limited, as indicated by observed clockwise hysteresis in bed-load transport rates during each spring snowmelt and by the coarse surface armor observed at sites. Flows above bankfull discharge move 37% of the bed load, whereas flows between mean annual discharge and bankfull move 57% of the bed load. The bed-load effective discharge has a recurrence interval that averages 1.4 yr and the magnitude of effective discharge averages 80% of bankfull discharge. The recurrence interval of bankfull discharge averages 2.0 yr. The ratio of effective discharge to bankfull discharge is independent of basin size, grain size, and gradient, although the ratio increases with the relative magnitude of large infrequent events.

ANNUAL HYSTERESIS IN BED LOAD RATING CURVES

The relationship between flow and bed load transport measured for 10 years in six gravel-bed streams in Idaho exhibits annual hysteresis. At a given flow rate, more bed load is carried by discharges preceding the first annual occurrence of a “threshold” rate, which is characteristic of each stream. Incorporating the effect of hysteresis leads to a small improvement in the fit of the bed load–flow regression. As the turning point for hysteresis, a constant threshold discharge is found to work better than the annual peak discharge. This bimodal hysteresis model is also found to out perform one with a more gradual transition, based on cumulative discharge. These results are interpreted to reflect a buildup of readily moved sediment supplies during the low-flow periods from late summer to early spring, supplies which are then exhausted by rising springtime discharges up to the threshold. The threshold is greater than mean annual discharge and about one-half bank-full discharge.

Streamflow record extension using power transformations and application to sediment transport

To obtain a representative set of flow rates for a stream, it is often desirable to fill in missing data or extend measurements to a longer time period by correlation to a nearby gage with a longer record. Linear least squares regression of the logarithms of the flows is a traditional and still common technique. However, its purpose is to generate optimal estimates of each days discharge, rather than the population of discharges, for which it tends to underestimate variance. Maintenance-of-variance-extension (MOVE) equations [Hirsch, 1982] were developed to correct this bias. This study replaces the logarithmic transformation by the more general Box-Cox scaled power transformation, generating a more linear, constant-variance relationship for the MOVE extension. Combining the Box-Cox transformation with the MOVE extension is shown to improve accuracy in estimating order statistics of flow rate, particularly for the nonextreme discharges which generally govern cumulative transport over time. This advantage is illustrated by prediction of cumulative fractions of total bed load transport.

The geometric, sedimentologic and hydrologic attributes of spring-dominated channels in volcanic areas

Abstract In volcanic areas of Idaho, Oregon and Montana, a number of perennial streams emerge from single springs or zones of springs. Surface drainage areas to these springs can be very small, often much smaller than the recharge area of the springs. Channels downstream of springs are often straight, or if sinuous, without regularity to the pattern. Bars are absent or poorly defined, but islands or downed timber are common in the channel. Channel width-to-depth ratios are large relative to those of runoff-dominated channels. Downstream hydraulic geometry exponents are similar, but the exponents for width and velocity are greater in spring-dominated channels. Manning roughness values are relatively large. The bedsurface in gravel-bed spring-dominated streams is armored. Computations indicate that bed material is probably capable of moving at bankfull stage. The hydrograph of spring-dominated streams is damped as compared to runoff-dominated streams locally and elsewhere. Peak flows occur months after precipitation or snowmelt. Mean annual flow for spring-dominated streams averages 72% of the flood with a recurrence interval of 2 years; the mean annual flow for runoff-dominated channels averages 18% locally and 25% elsewhere. The 50-year flood averages 1.6 times the 2-year flood on the annual series while the corresponding value for runoff-dominated channels in the region is 2.5. The damped hydrograph of spring-dominated streams suggests that they are somewhat different from runoff-dominated channels in the relationship between water and sediment. In spring-dominated channels, 34% of sediment is transported by flows above the 2-year flood—less than is observed typically in runoff-dominated channels. The effective discharge is similar in magnitude to the 2-year flood.