Pebble Counts Research Articles

A comparison between different methods for determining grain distribution in coarse channel beds

The determination of grain size distribution in alluvial channels plays a crucial role in understanding fluvial dynamics and processes (e.g., hydraulic resistance, sediment transport and erosion, and habitat suitability). However, to determine an accurate distribution, tremendous field efforts are often required. Traditionally, the grain size distribution of channel beds have been obtained by manually counting a set of randomly selected stones (the “pebble count”). Based on this elementary principle, many authors have proposed different adaptations to overcome weaknesses and problems with the original method; with the development of digital technology, photographic methods have been developed in order to significantly reduce the time spent in the field. Two of these “image-assisted” methods include Automated Grain Sizing, AGS, and Manual Photo Sieving, MPS. In this study, AGS and MPS were applied under ideal laboratory conditions, to be used as reference, and in two field conditions with different degrees of difficulty in terms of visual determination of the grain size distribution; these included an artificial unlined channel and two natural mountainous streams. The results were compared with those obtained with the pebble-count method. In general, strong agreement between the methods was found when they were applied under favorable conditions (”the laboratory”), and the differences between the image-assisted and pebble count methods were similar to those found in previous studies. Despite being more time consuming, MPS was deemed preferable to AGS when conditions are not optimal; in these cases, the time spent on image elaboration significantly increased in the AGS method (approximately three-fold), but the estimation error of the median grain size decreased by approximately 37%. The use of image-assisted analysis has proven to be robust for characterizing sediment in watercourse beds and reducing fieldwork time, but because field conditions can significantly affect the accuracy of results, the method choice must be carefully considered.

Stratigraphic evolution of provenance in a thick conglomerate-dominated Carboniferous succession in the southern Isle Madame area, Nova Scotia, and paleogeographic implications

Thick conglomerate-dominated sedimentary successions of the Horton and lower Windsor groups in the southern Isle Madame area show provenance variations with stratigraphic position, based on pebble counts, petrographic observations, and facies trends throughout the conglomeratic units. These coarse clastic sediments were deposited in a transtensional basin that formed south of a splay of the Minas Fault Zone, now repeated by younger faults in the study area. A stratigraphic section 1300 m thick that includes the base of the Horton Group was derived mostly, if not entirely, from units in the Avalonian Mira terrane to the east. Similar provenance indications continue stratigraphically upward in this section and elsewhere on Isle Madame through the thick lower and central parts of the Horton Group. In the upper part of the Horton Group, an influx of high-grade metamorphic and deformed plutonic clasts is recorded in the conglomerates, and the percentage of this material continues to increase above a dark fine-grained interval and into the overlying Windsor Group. The metamorphic clasts strongly resemble local basement rocksexposed as belts between conglomeratic domains in Isle Madame, indicating that these deeper crustal rocks were unroofed within the former basin in the late Tournaisian, resulting in redirected drainage patterns. Paleocurrent and facies information suggest that the Mira terrane sources were located to the northwest at the time of deposition. Hence, paleogeographic reconstruction for the area involves not only unroofing of the deeper crustal rocks, but also dextral transcurrent movement to place the appropriate parts of the Mira terrane at the northwestern corner of exposed parts of the basin. This movement was along a subsequently deformed part of the Minas Fault Zone.

Hydraulic Interaction between Rock Cross Vane Stream Restoration Structures and a Bridge Crossing

The performance of a stream restoration project that incorporates a bridge crossing is evaluated within a 3-year monitoring period. A goal of the project was to alleviate and prevent future sediment aggradation within the waterway of a low-clearance bridge crossing. The stream restoration project included two rock cross vanes and stepped riprap and vegetation bank stabilization. Monitoring of the project involved the collection of channel survey data, pebble counts, and general observations of instream structure condition and sediment movement. The evaluated performance of the restoration structures is related to the general hydrologic conditions, the historical changes in the watershed and channel, and the hydraulic conditions created by the low-clearance bridge crossing. Backwater effects created by the bridge crossing are found to be a substantial cause of the failure of the stream restoration project to meet its goals. The low-clearance bridge hydraulics are preventing a rock cross vane located upstream of the bridge from creating a scour hole in the centre of the channel; instead, aggradation is occurring in this portion of the channel. However, degradation is occurring downstream of the bridge causing the failure of the second rock cross vane and of the riprap and vegetation bank. Although the hydraulic conditions may stem from the initial design of the bridge crossing, any restoration structure should be designed according to the current site hydraulics. In addition to providing insight into the design and construction of stream restoration structures, the results have implications for the design and management of bridge crossings. Copyright © 2014 John Wiley & Sons, Ltd.

An Evaluation of Data Collected by Middle School and College-Level Students in Stream Channel Geomorphic Assessment

This project tested the accuracy and repeatability of geomorphic stream channel assessments conducted by two different middle school classes from the Walt Morey Middle School in Troutdale, OR and college students from Portland State University in Portland, OR. Each group surveyed the same three cross-sections in Fairview Creek, a tributary to the Lower Columbia River, in order to assess stream channel geometry, discharge, composition of the bed material, and water quality. The three student groups were all able to accurately document the stream channel geometry, including stream width and mean depth, indicating that these data can be successfully collected by volunteers of various ages. However, stream velocity obtained using the float method was consistently overestimated leading to a biased calculation of discharge, and the low precision of the measurements did not allow for a correction of the bias. The median particle size of the bed material determined by a pebble count was also overestimated by each group, but the low precision also negated the possibility of correcting the estimate. The stored fine sediment in the bed was underestimated by each group and again with low precision. The temperature, pH, and conductivity measured with a calibrated multimeter were accurate and precise for all groups.

Nesting habitat use by river chubs in a hydrologically variable Appalachian tailwater

AbstractAs hydrologic alteration continues to affect aquatic biodiversity, knowledge of the spawning requirements of fishes, especially ‘keystone’ or ‘foundation’ species, is critical for conservation and management. The objectives of this study were to quantify the spawning micro‐ and mesohabitat use of river chub Nocomis micropogon, a gravel mound nesting minnow, in a hydrologically regulated river in North Carolina, USA. At the microhabitat scale, substrate sizes on nests were compared with pebble counts in 1‐m2 adjacent quadrats. Average depths and current velocities at nests were compared with measurements from paired transects. At the mesohabitat scale, generalised linear mixed models (GLMMs) were used to identify the importance of average bed slope, average depth and percentages of rock outcrops (a measure of flow heterogeneity and velocity shelters) for predicting nest presence and abundance. To relate nesting activities to hydrologic alteration from dam operation, nest dimensions were measured before and after a scheduled discharge event approximately six times that of base flow. Additionally, linear regression was used to predict changes in the use of flow refugia and overhead cover with increased fluvial distance from the dam. Microhabitats in which nests were placed had, on average, slower current velocities and shallower depths. Gravel diameters of nests were significantly smaller than substrate particles adjacent to nests. GLMMs revealed that mesohabitats with nests were shallower, had more moderate slopes and greater proportions of rock outcrops than mesohabitats without nests. The scheduled discharge event significantly flattened nests. Near the dam, nests were built in close proximity (≤2 m) to velocity shelters; this relationship diminished with distance from the dam. River chubs are spawning habitat specialists. Because multiple species rely on river chub nests for reproduction and food, the needs of this species should be considered when managing instream flows.

Beaver dams and channel sediment dynamics on Odell Creek, Centennial Valley, Montana, USA

Beaver dams in streams are generally considered to increase bed elevation through in-channel sediment storage, thus, reintroductions of beaver are increasingly employed as a restoration tool to repair incised stream channels. Here we consider hydrologic and geomorphic characteristics of the study stream in relation to in-channel sediment storage promoted by beaver dams. We also document the persistence of sediment in the channel following breaching of dams. Nine reaches, containing 46 cross-sections, were investigated on Odell Creek at Red Rock Lakes National Wildlife Refuge, Centennial Valley, Montana. Odell Creek has a snowmelt-dominated hydrograph and peak flows between 2 and 10m3s−1. Odell Creek flows down a fluvial fan with a decreasing gradient (0.018–0.004), but is confined between terraces along most of its length, and displays a mostly single-thread, variably sinuous channel. The study reaches represent the overall downstream decrease in gradient and sediment size, and include three stages of beaver damming: (1) active; (2) built and breached in the last decade; and (3) undammed. In-channel sediment characteristics and storage were investigated using pebble counts, fine-sediment depth measurements, sediment mapping and surveys of dam breaches. Upstream of dams, deposition of fine (≤2mm) sediment is promoted by reduced water surface slope, shear stress and velocity, with volumes ranging from 48 to 182m3. High flows, however, can readily transport suspended sediment over active dams. Variations in bed-sediment texture and channel morphology associated with active dams create substantial discontinuities in downstream trends and add to overall channel heterogeneity. Observations of abandoned dam sites and dam breaches revealed that most sediment stored above beaver dams is quickly evacuated following a breach. Nonetheless, dam remnants trap some sediment, promote meandering and facilitate floodplain development. Persistence of beaver dam sediment within the main channel on Odell Creek is limited by frequent breaching (<1–5years), so in-channel sediment storage because of damming has not caused measurable channel aggradation over the study period. Enhanced overbank flow by dams, however, likely increases fine-grained floodplain sedimentation and riparian habitat. Contrasts between beaver-damming impacts on Odell Creek and other stream systems of different scales suggest a high sensitivity to hydrologic, geomorphic, and environmental controls, complicating predictions of the longer-term effects of beaver restoration.

Sediment changes upstream and downstream of the Altnahinch Dam in the River Bush in Northern Ireland: a preliminary study

The River Bush must reach a standard of good ecological potential (GEP) by 2015 due to the requirements of the water framework directive. The role of sediments within a water body is extremely important to all aspects of a river’s regime. The aim of this research is to investigate the effects of Altnahinch Dam on sediment distribution in the River Bush (a heavily modified water body) with comparison made against the Glendun River (an unmodified water body). Samples collected from the rivers were analysed by physical (pebble count, sieve analysis) and statistical methods (ANOVA, GRADISTAT). An increase in fine sediments upstream of the dam provides evidence that the dam is impacting sediment distribution. Downstream effects are not shown to be significant. The output of this study also implies similar impacts at other drinking water storage impoundments. This research recommends that a sediment management plan be put in place for Altnahinch Dam and that further studies be carried-out concentrating on fine sediment distribution upstream of the dam.

Differentiating the relative importance of land cover change and geomorphic processes on fine sediment sequestration in a logged watershed

Timber harvest often results in accelerated soil erosion and subsequent elevated fine (<2mm) sediment delivery to channels causing deleterious effects to numerous aquatic species, particularly salmonid fishes. Here we determine, through sediment physical analyses (pebble counts, embeddedness surveys, and interstitial shelter space counts) and geochemical analyses (7Be and 210Pbex activities), the amount and timing of delivery of fine sediment currently found on streambeds of the Narraguagus River watershed in coastal Maine. The role of recent timber harvest, documented via aerial photo spatial analysis, on fine sediment delivery is contrasted with the ability of the glacially influenced topography and surficial geology to deliver fine sediment to streams and to influence channel substrate. Results show that of the land use and geomorphic variables examined, only 210Pbex activities were significantly correlated with the amount of upstream harvest (r2=0.49). Concurrently, we find that unit stream power (particularly the slope component) explains much of the variability in channel substrate and that slope and stream power are largely influenced by the legacy of Pleistocene glaciation on channel form. Results suggest a conceptual model whereby fine sediment delivery as a result of late twentieth century timber harvest is likely dampened because of the low gradient landscape of coastal Maine. While geochemical tracers indicate recent fine sediment delivery in harvested areas, channels are likely capable of quickly winnowing these fines from the channel bed. These results further suggest that under contemporary land use conditions, the geomorphic and geologic setting represents a first-order control on channel substrate and habitat suitability for salmonid fishes, including federally endangered Atlantic salmon (Salmo salar), in coastal drainages of northeastern Maine.

Restoration Monitoring on the McKenzie River, Oregon

In the spring of 2012, we, the Stream Stewardship Team from the University of Oregon’s Environmental Leadership Program (ELP), conducted post-monitoring surveys at a side channel of the Middle McKenzie River (side channel 4) to compare with baseline monitoring data collected by the 2011 ELP Restoration Stewardship Team. The goal of this restoration project was to enhance juvenile spring Chinook salmon (Oncorhynchus tshawytscha) rearing habitat within the channel. In 2011 the U.S. Forest Service placed large woody debris (LWD) in five sections of the channel after baseline monitoring to increase the complexity of the streambed within the channel and to create a distribution of sediment optimal for salmon spawning habitat. We conducted pebble counts, cross-channel surveys, and a longitudinal profile of the stream to observe changes since the addition of LWD. Median pebble size decreased downstream of the LWD placements at gravel count 1 and increased upstream at gravel count 2. The percent of embedded sediment decreased at both gravel count sites. We also detected noticeable changes in the stream morphology at four of the five cross-sectional surveys as well as along the longitudinal profile. Sediment size distribution and the formation of pools at the downstream end of the channel showed an initial change in stream morphology since 2011, but further monitoring is warranted in order to fully assess the effects of LWD on streambed complexity and salmon spawning habitat.

Reply to Bunte et al. (2011) “Discussion of Daniels and McCusker (2010): Operator bias characterizing stream substrates using Wolman pebble counts with a standard measurement template.” Geomorphology 115, 194–198

Reply to Bunte et al. (2011) “Discussion of Daniels and McCusker (2010): Operator bias characterizing stream substrates using Wolman pebble counts with a standard measurement template.” Geomorphology 115, 194–198

Discussion of Daniels and McCusker (2010): “Operator bias characterizing stream substrates using Wolman pebble counts with a standard measurement template.” Geomorphology 115, 194–198

We feel that the presence of misinterpretations and omissionsregarding pebble count utility and application in Daniels andMcCusker (2010) warrants a response. Claiming to have done whatthe authors call a “standard Wolman pebble count,” the article byDanielsandMcCusker(2010)cangivethefalseimpressionthatuseofa template (or gravelometer) does not contribute toward reducingoperator variability. Looking at the study in detail shows that whilethe authors minimized errors in particle size measurement, they didnot minimize the particle selection bias. The authors conflatesampling and measurement error; accurate particle measurementusingtemplatesisof littlevalueif theparticlesbeingmeasured haveabiasintheirselection.Infact,thehighoperatorvariabilityobservedbyDaniels and McCusker's pebble counts is a good example of whathappens when one uses a particle selection technique that does notminimize operator bias in particle selection.The Daniels and McCusker study sites comprised mainly gravel,cobbles, and boulders. Traversing a bed with measured paces or heel-to-toe walks and blindly picking the first particle touched with apointed finger at the tip of the boot—as done by Wolman (1954) whoworked on streambeds with mostly mid-sized gravels—is difficult ifnot dangerous for an operator on very coarse beds (Bunte and Abt,2001a,b). To circumvent problematic pacing or heel-to-toe walks,Daniels and McCusker applied a nonstandard grab technique forparticleselection.Theydividedtheirrifflesamplingareasinto100gridcells each. Information on stream width and cell area size was notprovided but may have been a square foot in the ~10-ft-wideSchoolhouse Brook and between 0.25 and 1 m

Local sorting, bend curvature, and particle mobility in meandering gravel bed rivers

Hydraulic, grain scale sorting of mixed bed sediment influences the mobility of grains in discrete areas of river channels. To assess this effect, local values of surface grain size sorting were compared with measurements of bed load at corresponding locations in a bend of the Colorado River in Rocky Mountain National Park (RMNP), and the distribution of local Shields stress through the reach was derived from a two‐dimensional flow model. With decreasing degrees of local sorting, the relative mobility of the fine‐ and coarse‐size fractions of the load appeared to decrease and increase, respectively. Furthermore, back‐calculated critical Shields stress values for sediment entrainment decreased with values of local sorting, particularly for the upstream portion of the reach where particles were more poorly sorted and coarse grains had higher relative exposure. To evaluate the pervasiveness of these and other patterns of sorting in gravel rivers, detailed field measurements of channel topography and surface grain size (317 pebble counts) were obtained for seven additional reaches of differing curvature (radius of curvature/width from 1 to 28) near the headwaters of the Colorado and Fall rivers in RMNP. Moderately curved and tight bends (radius of curvature/width ≤ 7) were significantly better sorted than comparatively straight reaches. Values of local sorting decreased with distance downstream for the majority of curved channels, reflecting a reduction in the standard deviation of surface grain sizes toward the lower end of the reach; this effect increased slightly with bend sharpness.

Comparison of Automated Image-Based Grain Sizing to Standard Pebble-Count Methods

This study explores the use of an automated image-based method for characterizing grain-size distributions (GSDs) of exposed open-framework gravels by comparing the GSDs measured with the image-based method to distributions obtained with two pebble-count methods. Selection of grains for the two pebble-count methods was carried out using a gridded sampling frame and the heel-to-toe Wolman walk method at six field sites. At each site, 500-particle pebble-count samples were collected with each of the two pebble-count methods and digital images were systematically collected over the same sampling area. For the methods used, pebble counts collected with the gridded sampling frame were assumed to be the most accurate representations of the true grain-size population. Therefore, results from the image-based method were compared to the grid-derived GSDs for accuracy estimates; comparisons between the grid and Wolman walk methods were conducted to give an indication of possible variation between commonly used methods for the particular field sites used in the study. The grain-size comparisons were made at two spatial scales. At the larger scale, results from the image-based method were integrated over the sampling area required to collect the 500-particle pebble-count samples. At the smaller sampling scale, the image derived GSDs were compared to those from 100-particle, pebble-count samples obtained with the gridded sampling frame. The comparison shows that the image-based method performed reasonably well on five of the six study sites. For those five sites, the image-based method slightly underestimated all grain-size percentiles relative to the pebble counts collected with the gridded sampling frame, but the method performed well in estimating the median grain size (the average bias for ψ 5 , ψ 50 , and ψ 95 was 0.07ψ, 0.04ψ, and 0.19ψ, respectively). The Wolman pebble-counts yielded coarser results than the pebble counts obtained with the gridded sampling frame, especially for the smaller percentiles (the average bias for ψ 5 , ψ 50 , and ψ 95 was 0.20ψ, 0.16ψ, and 0.04ψ, respectively). Oversegmentation of large pitted grains in the image-analysis procedures was identified as a leading cause for failure of the image-based method at one of the sites. It is likely that lower degrees of oversegmentation and physical particle orientation contributed to the slight underestimation of all grain-size percentiles in the image-based method.

Contingency table analysis of pebble lithology and roundness: A case study of Huangshui River, China and comparison to rivers in the Rocky Mountains, USA

Contingency table analysis of pebble lithology and roundness is an effective way to identify the source terrane of a drainage basin and to distinguish changes in basin size, piracy, tectonism, and other events. First, the analysis to terrace gravel deposited by the Huangshui River, northeastern Tibet Plateau, China, shows statistically contrasting pebble populations for the oldest terrace (T7, Dadongling, 1.2 Ma) and the youngest terraces (T0–T3, ≤ 0.15 Ma). Two fluvial processes are considered to explain the contrast in correlation between lithology and roundness in T7 gravel versus T0–T3 gravel: 1) reworking of T7 gravel into T0–T3 gravel and 2) growth in the size of the river basin between T7 and T0–T3 times. We favor growth in basin size as the dominant process, from comparison of pebble counts and contingency tables. Second, comparison of results from Huangshui River of China to three piedmont streams of the Rocky Mountains, USA highlights major differences in source terrane and history. Like Rocky Mountain piedmont gravel from Colorado examples, the Huangshui gravels show a preference (observed versus expected frequency) for rounded granite. But unlike Rocky Mountain gravel, Huangshui gravel shows a preference for angular quartzite and for rounded sandstone. In conclusion, contrasting behavior of lithologies during transport, not always apparent in raw pebble counts, is readily analyzed using contingency tables to identify the provenance of individual lithologies, including recycled clasts. Results of the analysis may help unravel river history, including changes in basin size and lithology.

Temporal and spatial scales of geomorphic adjustments to reduced competency following flow regulation in bedload-dominated systems

Because of the combined effects of reduced sediment transport capacity and competency following flow regulation, morphological changes are expected to occur in channels downstream from dams and, specifically, at tributary junctions where local inputs of water and sediment occur. Using a combination of historical aerial photographs, mainstem- and tributary-channel pebble counts, and HEC-RAS flow modeling for two watersheds in south-central VT, one unregulated and the other regulated since 1961, we document the time series of post-regulation channel narrowing and associated bar growth due to the influx of tributary sediment. Channel adjustments at regulated tributary junctions have been significant in ca. 50 years following impoundment, with channels downstream of the confluences narrowing over 15% after an initial ca. 20-year lag before the onset of accelerated narrowing. Moreover, flow modeling suggests that downstream of regulated confluences, the modern median grain size ( d 50) along the channel bed is immobile. No significant channel narrowing has occurred either above or below unregulated tributary junctions or on the mainstem upstream of regulated confluences. However, greater channel sediment fining is observed upstream of regulated confluences than above unregulated confluences. Thus, the primary mode of mainstem channel adjustment differs up- and downstream of regulated tributaries. These confluence effects have occurred where the tributary drainage area is only 0.2 times that of the mainstem, well below the threshold ratio of 0.6 required for significant geomorphic effects at unregulated confluences, highlighting the geomorphic scale shift of dams. Lastly, we evaluate the downstream length required for a river to recover from the impacts of impoundment and demonstrate that even distal locations are impacted by flow regulation. Unlike the impacts of flow regulation in the western US where channel incision and bar erosion predominate following impoundment, we find that in situations where bed incision is minimal and where sediment loads are low but bed caliber high, bar growth and channel narrowing are significant adjustments at tributary junctions following impoundment. Therefore, at our sites the effects of dams on reduced competency may be more profound than on reduced sediment transport capacity, highlighting the importance of geologic and geomorphic settings in understanding fluvial responses to impoundment.

Provenance of fluvial terrace sediments within the Waipaoa sedimentary system and their importance to New Zealand source-to-sink studies

Late Pleistocene fluvial terrace deposits within the Waipaoa River system of North Island, New Zealand provide some insight into the fluvial history of the region that can be potentially translated to the marine sedimentary record. Pebble counts and sand detrital modes from terrace remnants suggest significant compositional differences between these terraces and the modern stream deposits. The terrace deposits are enriched in sandstone lithics (sand and gravel), as well as monomineralic quartz and feldspar (sand). In contrast, the modern Waipaoa River is dominated by mudstone fragments, mainly products of gullying in the Waipaoa River headwaters region. Both the terrace and modern river sand show a compositional dependency on grain size, with increasing lithic components in the coarse sand fractions. Carbonate lithic content and proportions of lithic types in the modern river and terrace sediments can be related to the distribution of calcareous geological units across the region. For example, the calcareous lithics (sand and gravel) in the Mangatu River, a major tributary of the Waipaoa, are derived from deep-water facies (allochthon) that dominate the catchment. These carbonate facies are petrographically distinct from the marl (calcareous mudstone) that dominates elsewhere in the Waipaoa River system, such as the Pliocene units in the Waimata River catchment. The combined petrologic data sets from the modern rivers and terraces in the Waipaoa River system provide a model to interpret the provenance of sandy deposits in the offshore marine realm.

Operator bias characterizing stream substrates using Wolman pebble counts with a standard measurement template

This study characterizes the variability associated with using a standard measurement template to perform Wolman pebble counts of coarse stream substrates. Four 10-count and two 12-count replicate experiments were conducted by five operators at three sample sites within gravel-bed streams using a gravel template. Results show significant differences between different operators sampling the same location, as observed in previous studies conducted without a standard sampling template, as well as significant variability within each operator's replicate samples. Variability was highest in clast estimates larger than D50 suggesting that these metrics cannot be consistently derived from Wolman pebble count samples. Variability was also highest in the low order study site suggesting that the sedimentary characteristics of low order streams increase the chance for operator bias. In light of our findings, we suggest that traditional 100-particle Wolman counts be utilized with more caution than is the current norm.

Comparison of Three Pebble Count Protocols (EMAP, PIBO, and SFT) in Two Mountain Gravel‐Bed Streams1

Abstract: Although the term ``pebble count'' is in widespread use, there is no standardized methodology used for the field application of this procedure. Each pebble count analysis is the product of several methodological choices, any of which are capable of influencing the final result. Because there are virtually countless variations on pebble count protocols, the question of how their results differ when applied to the same study reach is becoming increasingly important. This study compared three pebble count protocols: the reach‐averaged Environmental Monitoring and Assessment Program (EMAP) protocol named after the EMAP developed by the Environmental Protection Agency, the habitat‐unit specific U.S. Forest Service’s PACFISH/INFISH Biological Opinion (PIBO) Effectiveness Monitoring Program protocol, and a data‐intensive method developed by the authors named Sampling Frame and Template (SFT). When applied to the same study reaches, particle‐size distributions varied among the three pebble count protocols because of differences in sample locations within a stream reach and along a transect, in particle selection, and particle‐size determination. The EMAP protocol yielded considerably finer, and the PIBO protocol considerably coarser distributions than the SFT protocol in the pool‐riffle study streams, suggesting that the data cannot be used interchangeably. Approximately half of the difference was due to sampling at different areas within the study reach (i.e., wetted width, riffles, and bankfull width) and at different locations within a transect. The other half was attributed to using different methods for particle selection from the bed, particle‐size determination, and the use of wide, nonstandard size classes. Most of the differences in sampling outcomes could be eliminated by using simple field tools, by collecting a larger sample size, and by systematically sampling the entire bankfull channel and all geomorphic units within the reach.

Provenance and U–Pb geochronology of the Upper Cretaceous El Chanate Group, northwest Sonora, Mexico, and its tectonic significance

The Upper Cretaceous El Chanate Group, northwest Sonora, Mexico, is a 2.8 km thick clastic sedimentary sequence deposited in a continental basin closely related to volcanic activity. It consists of three formations: the Pozo Duro (oldest), the Anita, and the Escalante (youngest). Petrographic study, conglomerate pebble counts, and U–Pb geochronology of detrital zircons were performed to determine the source and age of this sequence, and to interpret its tectonic setting. In the sandstones of all three formations, the most abundant grains are those of volcanic composition (Q38F22L40, Q35F19L46, and Q31F22L47, respectively). The Pozo Duro Formation includes well-rounded quartz-arenite clast conglomerates, whereas conglomerates of the two upper units have clasts predominantly of andesitic and rhyolitic composition. The most likely source for these sediments was the Jurassic volcanic arc exposed in northern Sonora and southern Arizona. Zircons from five sandstone samples define two main age groups, Proterozoic and Mesozoic. The first ranges mostly from 1000 to 1800 Ma, which suggests the influence of a cratonic source. This zircon suite is interpreted to be recycled and derived from the same source area as the quartz-rich sandstone clasts in the basal part of the section. Mesozoic zircons range from Triassic to Late Cretaceous, which confirms the proposed Late Cretaceous age for the sequence, and also corroborates Jurassic felsic source rocks. Another possible source was the Alisitos volcanic arc, exposed along the western margin of the Baja California Peninsula. Of regional significance is the great similarity between the El Chanate Group and the McCoy Mountains Formation of southeastern California and southwestern Arizona. Both are Cretaceous, were deposited in continental environments, and have similar zircon-age patterns. Also, both exhibit intense deformation and locally display penetrative foliation. These features strongly suggest that both units underwent similar tectonic histories.

Monitoring the channel process of a stream restoration project in an urbanizing watershed: a case study of Kelley Creek, Oregon, USA

AbstractPacific Northwest (PNW) streams in the United States were impacted by the 20th century development, when removal of instream structure and channelization degraded an aquatic habitat. The lower Kelley Creek in southeast Portland, USA was channelized during the 1930's Works Progress Administration (WPA) projects. Stream restoration reintroduced pool‐riffle sequences and heterogeneous substrates to protect salmonids while mitigating impacts from flooding. We investigated whether the restored pool‐riffle morphology changed substantially following effective discharge events. We examined channel forms for four reaches representing three time periods—pre‐development (two reference reaches), development and restoration. We conducted thalweg profiles, cross‐sections and pebble counts along the reaches to examine how channel geometry, residual pool dimensions and particle size distribution changed following effective discharge events. The effective discharge flows altered the restoration reach more substantially than the reference reaches. The restoration reach decreased in median particle size, and its cross‐sectional geometry aggraded near its margins. However, the residual pool morphology remained in equilibrium. Richardson Creek's reference reach degraded at the substrate level, while Kelley Creek's reference reach remained in equilibrium. The restoration reach's aggradation may have resulted from sedimentation along the nearby Johnson Creek. In contrast, Richardson Creek's degradation occurred as upstream land use may have augmented flows. Stream channels with low gradient pool‐riffle morphologies are ideal for salmonid spawning and rearing and should be protected and restored within urban corridors. The findings of our study suggest that the connectivity of streams and the dynamic fluvial geomorphology of stream channels should be considered for stream restoration projects in humid temperate climates. Copyright © 2008 John Wiley &amp; Sons, Ltd.