Philip N. Owens

Fingerprinting suspended sediment sources in the catchment of the River Ouse, Yorkshire, UK

Statistically verified composite fingerprints and a multivariate mixing model have been employed to establish the main sources of the suspended sediment transported through the lower, non-tidal reaches of the River Ouse and one of its main tributaries, the River Wharfe, during the period 1994–1997. In the case of the suspended sediment samples collected from the River Ouse, the load-weighted mean contributions from uncultivated topsoil, cultivated topsoil and channel bank sources were estimated to be c. 25, 38 and 37%, respectively, while for the River Wharfe these sources contributed c. 70, 4 and 23%, respectively (c. 4% was derived from woodland topsoil). Suspended sediment samples collected during higher flows evidenced a greater contribution from channel banks than samples collected during lower flows. Source materials were also differentiated according to the three main geological source areas (Carboniferous, Permian and Triassic, and Jurassic) and their load-weighted mean contributions were estimated to be c. 24, 41 and 35% for the River Ouse and c. 91, 9 and 0% (there are no Jurassic rocks in this catchment) for the River Wharfe, respectively. When suspended sediment samples from tributary streams were used to characterize each geological source area, the equivalent results for the River Ouse were c. 30, 46 and 24%. Considering the three main tributaries that contribute to the River Ouse, the load-weighted mean contributions from the rivers Swale, Ure and Nidd were estimated to be 82, 15 and 3%, respectively. These values have been compared with estimates of the relative magnitude of the annual suspended sediment loads of these three rivers for the years 1995 and 1996 derived from continuous monitoring of discharge and turbidity. Differences between the two sets of results are ascribed to the different periods of record involved and to the timing of suspended sediment sampling relative to the overall storm hydrograph, and thus the degree to which the available samples are representative of the overall suspended sediment flux. Although a number of limitations must be recognized, the fingerprinting approach to source ascription is seen as providing valuable information regarding suspended sediment sources in the study catchments. Copyright

Storage of sediment-associated nutrients and contaminants in river channel and floodplain systems

Abstract Samples of fine-grained channel bed sediment and overbank floodplain deposits were collected along the main channels of the Rivers Aire (and its main tributary, the River Calder) and Swale, in Yorkshire, UK, in order to investigate downstream changes in the storage and deposition of heavy metals (Cr, Cu, Pb, Zn), total P and the sum of selected PCB congeners, and to estimate the total storage of these contaminants within the main channels and floodplains of these river systems. Downstream trends in the contaminant content of the

The phosphorus content of fluvial sediment in rural and industrialized river basins

The phosphorus content of fluvial sediment (suspended sediment and the < 63 microm fraction of floodplain and channel bed sediment) has been examined in contrasting rural (moorland and agricultural) and industrialized catchments in Yorkshire, UK. The River Swale drains a rural catchment with no major urban and industrial areas, and the total phosphorus (TP) content of fluvial sediment is generally within the range 500-1,500 microg g(-1). There is little evidence of any major downstream increase in TP content. In contrast, fluvial sediment from the industrialized catchments of the Rivers Aire and Calder exhibits both higher levels of TP content and marked downstream increases, with values of TP content ranging from < 2,000 microg g(-1) in headwater areas upstream of the main urban and industrial areas, to values > 7,000 microg g(-1) at downstream sites. These elevated levels reflect P inputs from point sources, such as sewage treatment works (STWs) and combined sewer overflows. The influence of STWs is further demonstrated by the downstream increase in the inorganic P/organic P ratio from < 2 in the headwaters to > 4 in the lower reaches. Comparison of the P content of suspended sediment with that of the <63 microm fraction of potential source materials suggests that topsoil from upland moorland/pasture and from cultivated areas, and channel bank material are likely to be the main sources of particulate P (PP) in the River Swale and in the headwaters of the Rivers Aire and Calder. In the middle and lower reaches of the Rivers Aire and Calder, inputs associated with urban and industrial land uses, such as STWs, industrial effluents and street dust, are likely to represent the dominant sources of PP. During high flow events, such urban inputs may be diluted by inputs from moorland and agricultural land in the headwaters. Consequently, for all three rivers, there are inverse relationships between the TP content of suspended sediment and both discharge and suspended sediment concentration, reflecting changes in sediment and P sources during high flow events. Spatial variations in the P contents of the < 63 microm fraction of overbank floodplain deposits and channel bed sediment evidence a similar pattern as those for suspended sediment, with relatively low levels of TP in the River Swale and elevated levels in the middle and downstream reaches of the Rivers Aire and Calder. The PP concentrations associated with floodplain and channel bed sediment are, however, lower than equivalent values for suspended sediment, and this primarily reflects the differences in the particle size composition between the three types of sediments. Rates of floodplain deposition and the amounts of fine-grained sediment stored in the river channels are relatively high, and suggest that such environments may represent important sinks for PP. Based on the sediment samples collected from the study basins, a simple four-fold classification which relates the TP content of suspended sediment to upstream land use has been established. Both the range and the absolute values of TP content tend to increase with an increase in the level of urbanization and industrialization.

The role of channel and floodplain storage in the suspended sediment budget of the River Ouse, Yorkshire, UK

This study attempts to quantify the amount of fine-grained (ca. < 150 Ixm) sediment stored on the floodplains and on the channel bed of the non-tidal sections of the main channels in the catchment of the River Ouse (3315 km 2) and of one of its tributaries, the River Wharfe (818 km2), in Yorkshire, UK. Caesium-137 analyses of floodplain sediment cores were used to quantify the amount of floodplain deposition as a result of overbank flooding during the last ca. 40 years. A combination of bulk and sectioned cores were collected along transects perpendicular to the channel at 26 sites throughout the study basins. In general, rates of overbank sedimentation decrease with distance from the channel. The average values for individual transects range between 0.010 and 0.554 g cm -2 year -1. Floodplain storage along the main channels of the Ouse and Wharfe basins accounts for 60645 and 10325 t year -1, respectively, and represents a net loss from the system. The amount of fine-grained sediment stored on the channel bed was estimated by a survey undertaken in August 1996, during which the fine material deposited on the bed was resuspended and its mass estimated at 16 locations. The average values for the individual locations range between 0.017 and 0.924 g cm -2 and tend to increase downstream. The total channel bed storage at the time of sampling in 1996 was estimated to be 16076 and 1866 t for the Ouse and Wharfe basins, respectively. It is assumed that channel bed storage is seasonal and that no net loss to the system occurs at the annual timescale. Floodplain storage for the Ouse and Wharfe basins represents 39 and 49%, and channel bed storage equals 10 and 9%, respectively, of the annual suspended sediment load (1995-1996) delivered to the channel system. These results have important implications for the routing of fine-grained sediment and sediment-associated contaminants in drainage basins, and for the interpretation of downstream sedimev~t yields in terms of upstream sediment mobilisation.

Use of floodplain sediment cores to investigate recent historical changes in overbank sedimentation rates and sediment sources in the catchment of the River Ouse, Yorkshire, UK

Abstract Floodplain sediment cores collected from seven sites in the catchment of the River Ouse, in Yorkshire, UK, have been used to provide information on recent historical changes in both rates of overbank sedimentation and sediment sources. The environmental radionuclides 137 Cs and unsupported 210 Pb have been used to establish chronologies for each core and to estimate average sediment accumulation rates for the last ca. 30 and 100 years, respectively. Average sedimentation rates for the individual cores ranged from 0.11 to 1.04 g cm−2 yr−1. In all but one case, the estimates of average sedimentation rate during the last ca. 30 years for the individual cores are broadly similar to those for the last ca. 100 years, suggesting that overbank sedimentation rates have been essentially uniform over the longer time period. Composite fingerprints, based on a combination of geochemical and mineral magnetic properties, and a numerical mixing model have been used to investigate downcore changes in sediment source. In the case of source type, most of the cores reflect a primarily topsoil source, although there have been periods with increased contributions from subsoil/channel bank sources. Within the Ouse basin in general, the period commencing in the late 19th and early 20th century and extending through to the 1960s, was characterised by increased contributions from topsoil sources. However, contributions from subsoil/channel bank sources have increased over the last few decades. The source tracing results relating to sediment contributions from the three main geological/topographic zones are in broad agreement with the proportion of the area of the catchment underlain by each rock type. Temporal variations in the contributions from the three geological/topographic zones vary from site to site, but for the lower reaches of the River Ouse contributions from areas underlain by Permian and Triassic rocks, which mainly outcrop in the Vale of York, have increased since the turn of the century. The changes in sediment source identified are probably a reflection of changes in land use and management (and possibly changes in climate). These results enable estimates of contemporary suspended sediment fluxes and sources to be placed into a historical context and provide a means of considering the likely impact of potential future changes in land use and climate in the study basin.

Spatial variability of caesium-137 inventories at reference sites: an example from two contrasting sites in England and Zimbabwe

Bomb-derived caesium-137 (137Cs) fallout has been widely used to investigate rates and patterns of soil erosion and sediment deposition. Within this approach considerable importance is placed on establishing the local total 137Cs fallout or inventory at a reference site. However, relatively little information currently exists concerning the variability of 137Cs inventories at reference sites. This paper reports an investigation of the spatial variability of 137Cs inventories at two contrasting reference sites at Yendacott, England and Msana, Zimbabwe. Replicate bulk soil cores were collected at the intersections of a grid, yielding 60 and 27 samples from Yendacott and Msana, respectively. Although the total variability may be derived from four sources (random spatial, systematic spatial, sampling and measurement), the observed variability was in both cases primarily due to random spatial variations in 137Cs inventories. The reference inventory range (at the 95% confidence level) was 205–230 and 17.0–33.4 mBq cm−2 for the Yendacott and Msana sites, respectively.

Fingerprinting suspended sediment sources in a large urban river system

Very few studies have attempted to quantify the sources of suspended sediment transported in urban river systems. In this study, statistically verified composite fingerprints and a multivariate mixing model have been used to identify the main sources of the suspended sediment transported by the River Aire and its main tributary, the River Calder. Because of the polluted nature of the Aire/Calder catchment and its effect on fingerprint property concentrations, source tracing was undertaken separately for the upper and lower reaches. The mean contributions from individual source types (i.e. surface materials from woodland, uncultivated and cultivated areas, channel bank material, road dust and solids from sewage treatment works) varied between the upper and lower reaches of the rivers, reflecting the change in land use from primarily pasture and moorland in the upper reaches to mainly urban areas (with some cultivated land) in the lower reaches. The suspended sediment in the upper reaches of the River Aire originates largely from channel bank sources (43-84%) and from uncultivated topsoil (16-57%). In the lower reaches of the Aire/Calder system, local sources of cultivated topsoil contribute 20-45% of the suspended sediment load and there is a significant contribution from urban sources, such as road dust (19-22%) and solids from sewage treatment works (14-18%). In the upper reaches, the proportion of sediment derived from each of the two main geological areas corresponds broadly to the proportion of the catchment occupied by each geological area. The relative contribution from the Rivers Aire and Calder to the suspended sediment load transported below the confluence demonstrates that most of the sediment is derived from the River Calder.

The particle size characteristics of fluvial suspended sediment in the Humber and Tweed catchments, UK.

This paper presents information on the absolute (chemically-dispersed) particle size characteristics of the suspended sediment transported by rivers in the Humber and Tweed basins during the period 1994-1998. For most of the rivers, > 95% of the suspended sediment load at the time of sampling was < 63 microm (i.e. silt- and clay-sized material) and < 5% was > 63 microm (i.e. sand-sized material). The < 2 microm fraction (i.e. clay-sized material) typically ranged between 15 and 25%. The average median (d50) particle size for the individual rivers ranged between 4.1 and 13.5 microm. Generally, the particle size characteristics of suspended sediment transported in the two basins were similar. There were, however, noticeable spatial variations in the particle size composition of suspended sediment within the study basins, which reflected the particle size of the sediment sources and their spatial variation, and the selectivity of the sediment mobilization and delivery processes. When particle size parameters were plotted against discharge, there were no significant relationships, although there was some evidence of trends varying between sites. The lack of significant relationships with discharge reflects the fact that sediment particle size is largely supply-controlled, rather than a function of flow and hydraulics. When particle size variations were examined during individual storm events, there was evidence of a pulse of coarse sediment on the rising limb of the hydrograph. This may reflect the remobilization of coarse channel bed sediment as flow velocity and shear stress increase. Finer sediment was transported subsequently during the hydrograph peak and on the falling limb. The findings reported have important implications for understanding and modelling suspended sediment, and associated contaminant, dynamics in river basins.

The behaviour of bomb-derived caesium-137 fallout in catchment soils

The use of 137Cs measurements for investigating soil erosion and sediment redistribution within the landscape is now well established. However, for this potential to be fully realised, and to interpret the results obtained from such investigations in a meaningful manner, it is important to understand how bomb-derived 137Cs fallout interacted with particles at the soil surface. Furthermore, as it is the surface soil which is mobilised by most wind and water erosion processes, it is important to determine how the 137Cs content at the surface has changed over time. This paper reports a series of field and laboratory experiments which were carried out to provide empirical information on the fate of 137Cs applied to different soil types to simulate fallout during individual storm events and over a 10 month period. The results suggest that most of the 137Cs was contained in the top few millimetres of the soil profile, with activity concentrations declining approximately exponentially with depth. A field sampling programme was carried out to examine the net longer-term effect of internal soil processes and of continuous 137Cs fallout on depth distributions and surface activity concentrations of radiocaesium by examining the 137Cs profiles associated with a representative range of soil types subject to different land use. The results show that in uncultivated soil most 137Cs is still contained in the top surface layers, while in cultivated soils the 137Cs is mixed throughout the plough layer. This information is then used to formulate a mathematical description of the behaviour of 137Cs fallout in topsoils that can be used to estimate 137Cs activity concentrations at the surface of uncultivated and cultivated fields from the start of atmospheric fallout to the present day.

Deposition and storage of fine-grained sediment within the main channel system of the River Tweed, Scotland

Thispaperassessestheimportanceofdepositionandstorageoffine-grained(c.< 150m)sedimentonthefloodplainsand bedsofthemain(non-tidal)channelsoftheRiverTweed(4390km 2 ),Scotland,andtwoofitstributaries(RiverTeviotand Ettrick Water). Caesium-137 analysis of floodplain sediment cores has been used to estimate average rates of overbank sedimentation during the last 30 to 40 years. Average values for individual transects ranged from 016 to 218 kg m ˇ2 a ˇ1 (013to2 2m ma ˇ1 ).Themeanforthe10transectsinvestigatedwas129kgm ˇ2 a ˇ1 (1 3m ma ˇ1 ).Thetotalamountoffine sediment deposited was estimated to be about 44 000 t a ˇ1 . The fine-grained sediment stored in the channel bed was quantifiedusingresuspensiontechniques.Averagevaluesforindividualsitesrangedfrom012to096kgm ˇ2 .Themeanfor the10sitesinvestigatedwas056kgm ˇ2 .Thetotalamountofsedimentstoredonthechannelbedofthemainchannelsystem at the time of sampling was estimated to be about 4300 t. Comparison of these estimates of floodplain and channel storage with the estimated suspended sediment loadfor the River Tweed at the downstream gauging site atNorham, indicates that floodplainsedimentationandchannelbedstoragerepresentabout40and4percent,respectively,oftheannualloadoffine sedimentdeliveredtothemainchannelsystem.Erosionofchannelbankswillreintroducetheequivalentofabout30percent of the floodplain-deposited sediment back into the channel. The residence time of the fine-grained sediment stored on the channel bed is probably less than one year, but that of sediment deposited on the floodplain is likely to be considerably longer. Conveyance losses associated with overbank deposition have important implications for the routing of sediment throughfluvialsystemsandtheinterpretationofdownstreamsedimentyields.Copyright#1999JohnWiley&Sons,Ltd.