John Pitlick

Variability of bed mobility in natural, gravel-bed channels and adjustments to sediment load at local and reach scales

Local variations in boundary shear stress acting on bed-surface particles control patterns of bed load transport and channel evolution during varying stream discharges. At the reach scale a channel adjusts to imposed water and sediment supply through mutual interactions among channel form, local grain size, and local flow dynamics that govern bed mobility. In order to explore these adjustments, we used a numerical flow model to examine relations between model-predicted local boundary shear stress (тj( and measured surface particle size (D50) at bank-full discharge in six gravel-bed, alternate-bar channels with widely differing annual sediment yields. Values of тj and D50 were poorly correlated such that small areas conveyed large proportions of the total bed load, especially in sediment-poor channels with low mobility. Sediment-rich channels had greater areas of full mobility; sediment-poor channels had greater areas of partial mobility; and both types had significant areas that were essentially immobile. Two reach-mean mobility parameters (Shields stress and Q*) correlated reasonably well with sediment supply. Values which can be practicably obtained from carefully measured mean hydraulic variables and particle size would provide first-order assessments of bed mobility that would broadly distinguish the channels in this study according to their sediment yield and bed mobility.

Observations of Flow and Sediment Entrainment on a Large Gravel-Bed River

Constant-discharge reservoir releases on the Trinity River, California, provide an unusual opportunity to unambiguously relate flow and gravel entrainment on a large gravel-bed river. Bed shear stress т0 was estimated using local observations of depth-averaged velocity. Gravel entrainment was measured using large tracer gravel installations. Lateral variability of т0 is large, even for straight channels with simple, trough-like geometry. No simple relation exists between local and cross-section mean values of т0 . Fine grains (less than 8 mm; 20–30% of the bed material) are transported at lower discharges than coarse grains. Scour to the base of the bed surface layer occurs at a dimensionless shear stress тg* ≈ 0.035, for тg* formed using local т0 and the median grain size of the gravel portion of the bed. The dimensionless reference transport rate W* = 0.002, often used as a surrogate for the threshold of grain motion, occurs at nearly the same тg*. At smaller тg*, entrainment and transport rates decrease rapidly, becoming vanishingly small at тg* ≈ 0.031. Even at very small gravel transport rates, all sizes are transported, although the coarsest sizes are in a state of partial transport in which only a portion of the exposed grains are entrained. Both entrainment and cumulative transport observations suggest that maximum scour depth for plane-bed transport is slightly less than twice the surface layer thickness.

FLOW–SEDIMENT–BIOTA RELATIONS: IMPLICATIONS FOR RIVER REGULATION EFFECTS ON NATIVE FISH ABUNDANCE

Alteration of natural flow regimes by river regulation affects fish distribution and assemblage structure, but causative pathways are not always direct and may go un- recognized. The Colorado River population of the endangered Colorado pikeminnow, Ptych- ocheilus lucius, suffers from low rates of recruitment and reduced carrying capacity. We hypothesized that availability of prey fish for this large-bodied native piscivore may, in part, be limited by reduced standing crops of periphyton and macroinvertebrates resulting from accumulation of fine sediment in the riverbed. We stratified the 373-km-long study area into 11 strata and sampled various physical and biological parameters in runs and riffles of three randomly selected 1- to 3-km-long study reaches in each stratum during base flows of spring and fall 1994-1995. Significant correlations were found between biomass of both chlorophyll a and macroinvertebrates and various physical metrics that described the degree of fine sediment accumulation in gravel-cobble substrates. Riffles were relatively free of fine sediment throughout the study area, but substrates of runs contained progressively more fine sediments with distance downstream. There was a cor- responding longitudinal change in biota along the river continuum with greatest biomass of fish, invertebrates, and periphyton upstream. Adult pikeminnow were concentrated in upstream strata where potential prey fishes were most abundant. We suggest that fine- sediment effects on biota have increased in recent years as a result of river regulation. Historically, spring snowmelt frequently produced flows with magnitudes sufficient to mo- bilize the bed and winnow silt and sand from coarse substrates. Following regulation, the mean recurrence interval of such flows lengthened from 1.3-2.7 yr (depending on the stratum) to 2.7-13.5 yr, extending the duration of fine sediment accumulation and potentially depressing biotic production. Our results describe and help explain the spatial distribution of the Colorado River fish community and establish a link between flow, sediment, and the riverine food web supporting the communitys top predator. To maintain intact native fish communities in this and other river basins, managers need to identify functional aspects of the natural hydrograph and incorporate these findings into river restoration efforts.

Relation between peak flows, precipitation, and physiography for five mountainous regions in the western USA

Abstract Flood frequency curves are presented for five regions in the western USA characterized by diverse climate but similar physiography. The regional curves are obtained via an index flood method based on probability-weighted moments of the generalized extreme value distribution. The curves differ markedly in their slopes, reflecting differences in mechanisms that produce floods. The Foothills region of Colorado is typical of semi-arid areas where floods are produced by intense thunderstorms. In this region, the 100-year flood may be more than ten times the mean annual flood. The Alpine region of Colorado is typical of areas where runoff is generated by snowmelt. In this region, the 100-year flood is less than two times the mean annual flood. The Sierra Nevada, Coast Range and Klamath Mountain regions of California typify areas where floods are produced by large-scale frontal storms. In these regions, the 100-year flood may be three to six times the mean annual flood. In all regions the mean annual flood is most highly correlated with drainage area and mean annual precipitation. No correlation is found between the standard deviation of annual maxima and various measures of basin physiography or drainage area. It is suggested that the variation in flood frequency distributions reflects largely the variability in precipitation amount and intensity rather than differences in physiography.

Formation of Martian gullies by the action of liquid water flowing under current Martian environmental conditions

[1] Geomorphic evidence suggests that recent gullies on Mars were formed by fluvial activity. The Martian gully features are significant because their existence implies the presence of liquid water near the surface on Mars in geologically recent times. Irrespective of the ultimate source of the fluid carving the gullies, we seek to understand the behavior of this fluid after it reaches the Martian surface. We find that contrary to popular belief, the fluvially carved Martian gullies are consistent with formation conditions such as now occur on Mars, outside of the temperature-pressure stability regime of liquid water. Our model of the action of flowing pure liquid water produces the observed gully length distribution only at surface pressures and temperatures below the triple point where liquid water simultaneously boils and freezes and thus suggests that gullies were formed under conditions similar to present-day Mars. Our results suggest a typical flow rate of 30 m 3 /s to carve the gully channels. At least 0.15 km 3 has flowed across the surface of Mars to carvethegullysystemsobservedtoday,andthiswouldrequireanaquifer recharge rate of � 10 � 13 –10 � 12 m/yr. The absence of gullies on Mars that are long enough to have been created above the triple point pressure argues that the atmospheric pressure has not been significantly larger than it is now since the origin of the gullies. This result may imply that Mars does not possess a significant reservoir of condensed CO2.

Geomorphology and endangered fish habitats of the upper Colorado River 1. Historic changes in streamflow, sediment load, and channel morphology

The hydrologic, geomorphic, and ecologic effects of reservoir operations are thought to be key factors in the decline of native fishes in the upper Colorado River basin. The present paper examines the extent to which changes in streamflow and sediment loads have affected alluvial reaches of the Colorado River near Grand Junction, Colorado. The analysis shows that since 1950, annual peak discharges of the Colorado River and its major tributary, the Gunnison River, have decreased by 29–38%. The total volume of runoff delivered to the study area has not changed significantly over the period of record, but the annual hydrograph has been modified greatly by reductions in peak flows and augmentation of base flows. Annual suspended sediment loads of the Colorado River and Gunnison River have likewise decreased. This was particularly apparent during the period from 1964 to 1978, when annual sediment loads were 40–65% less than the long-term average. Analysis of aerial photographs indicates that between 1937 and 1993 the main channel of the Colorado River has narrowed by an average of 20 m and about 1/4 of the area formed by side channels and backwaters has been lost.

Geomorphology and endangered fish habitats of the upper Colorado River: 2. Linking sediment transport to habitat maintenance

Alluvial reaches of the Colorado River near Grand Junction, Colorado, provide important habitat for the endangered Colorado squawfish. This paper examines recent changes in the geomorphology of the Colorado River and addresses the question of what can be done to improve existing fish habitats. Observations of channel change during periods of above-average runoff from 1993 through 1995 indicate minor scour and fill of the main channel, but more significant scour and enlargement of side-channel and backwater habitats and flushing of fine sediment from the bed. Modeled relations between discharge and dimensionless shear stress т* indicate that the threshold of bed load transport (т*=0.03) occurs at about half the bank-full discharge; these flows are exceeded about 26 days per year on average. The bank-full discharge produces an average т* of 0.047 and is exceeded about 8 days per year. The width and depth of the channel thus appear to be set by a discharge that produces an average boundary shear stress of ∼1.5 times greater than the critical shear stress. The effective discharge is slightly less than the bank-full discharge, and approximately 80% of the total sediment load is carried by the highest 10% of all discharges.

Response and recovery of a subalpine stream following a catastrophic flood

The July 15, 1982, Lawn Lake flood in Rocky Mountain National Park, Colorado, was caused by the failure of a 79-yr-old earthen dam. Peak discharges of the flood far exceeded naturally occurring flows, and it caused severe channel disturbance along most of Roaring River and some parts of Fall River. This study documents the geomorphic response of a 5-km reach of Fall River in the 5 yr following the flood. In 1983, the first year after the Lawn Lake flood, snowmelt flows were well above average. These high flows together with very high sediment yields from Roaring River resulted in significant geomorphic changes on reaches of Fall River downstream. During the 1983 snowmelt runoff, ∼15.5 x 106 kg of bed-load sediment was eroded from the upper part of the study area. These loads were at least 1,000 times higher than before the Lawn Lake flood. Most of this sediment was then deposited in a highly sinuous reach of Fall River in the lower part of the study area. This reach had not been much affected by the Lawn Lake flood, but sedimentation during the period of high flow in 1983 completely filled in the channel, resulting in the formation of a continuous 2.3-km-long depositional zone. In 1984, sediment yield from Roaring River declined dramatically, and this trend continued for the next 3 yr. By 1987, the bed-load sediment yield in the upper reaches of Fall River was only about 0.4 x 106 kg/yr. The decline in sediment loads resulted in progressive erosion and recovery of the original channel of Fall River in the depositional zone reach. Recovery in the upstream part of the depositional zone was complete by 1985. Recovery in the downstream part of the depositional zone took longer because of the continued supply of sediment and because the sediment was mobile less of the time. As of 1987, about 80% of the material initially stored in the sedimentation zone had been eroded. Bed-load sediment yields at a sampling site 1 km downstream of the terminus of the depositional zone ranged from 5 x 106 to 11 x 106 kg/yr, but showed no significant decline over the 5-yr study period. The average rate of bed-load transport through this reach was at least 100 times greater than before the Lawn Lake flood, but few discernible channel changes resulted from the higher loads.

Interparticle collision of natural sediment grains in water

Elastohydrodynamic theory and measurements of particle impacts on an inclined glass plane in water are used to investigate the mechanics of interparticle collisions in sediment-transporting flows. A collision Stokes number is proposed as a measure of the momentum of an interparticle collision versus the viscous pressure force in the interstitial gap between colliding particles. The viscous pressure force opposes motion of the particles on approach and rebound. A Stokes number of between 39 and 105 is estimated as the critical range below which particle impacts are completely viscously damped and above which impacts are partially elastic. The critical Stokes number is shown to roughly coincide with the Bagnold number transition between macroviscous and grain inertial debris flows and the transition between damped and partially elastic bed load transport saltation impacts. The nonspherical nature of natural particles significantly alters the motion of the center of mass after a partially elastic collision. The normal to the point of contact between the particles does not necessarily go through the center of mass. Thus normal rebound of the center of mass may not occur. A model of particle motion after rebound for particles of arbitrary shape, conserving both linear and angular momentum, is proposed.

In situ determination of particle friction angles of fluvial gravels

Particle friction angles Φ represent the physical resistance to initial movement of a sediment particle and are therefore useful for relating initiation of motion to particular flows. We determined over 8000 friction angle values at five natural rivers by applying a new method that uses a digital load cell to directly measure the force Fd required to pivot or slide a particle out of its natural resting place. Within each site, median Φ values were very similar to previously reported relations, yet different enough between sites that a location-general predictive empirical relation would produce errors in Φ of ±10 degrees for Di/Ks > 1. Furthermore, within a Di/Ks class at a given site the range in Φ was as large as 80°, much greater than the range of median values between classes for natural sediment mixtures. Using estimates of τc* from extensive bed load measurements made by Andrews and Erman [1986] at Sagehen Creek and the in situ Φ measurements made in this study together with a theoretical model developed by Wiberg and Smith [1987], we show that Φ measurements made with this new method can be used to accurately predict τc* for natural, water-worked sediments. Additionally, these results confirm that a Φ value ≪Φ50 is more appropriate for predicting τc* of a given size class.