Stream Dissolved Organic Carbon Concentrations Research Articles

Hydrological control of dissolved organic carbon dynamics in a forested headwater catchment, Kiryu Experimental Watershed, Japan

AbstractTo evaluate the influence of hydrological processes on dissolved organic carbon (DOC) dynamics in a forested headwater catchment, DOC concentration was observed along the flow path from rainfall to stream water via throughfall, soil water, groundwater, and spring water for 4 years, and DOC flux through the catchment was calculated. The spatial and temporal variations in DOC concentration and flux were compared with physical hydrological observations and the mean residence time of water. In the upslope soil layer, DOC concentrations were not significantly correlated with water fluxes, suggesting that DOC concentrations were not strictly controlled by water fluxes. In the upslope perennial groundwater, DOC concentration was affected by the change in the amount of microbial degradation of DOC produced by changes in the mean residence time of water. In stream water, the temporal variation in DOC concentration was usually affected by changes in DOC concentration of the inflow component via vertical infiltration from above the perennial groundwater. During dry periods, however, the component from inflow via vertical infiltration was negligible and DOC in the upslope perennial groundwater became the major component of stream water DOC. The temporal variation in stream water DOC concentration during baseflow was affected by rainfall patterns over several preceding months. Therefore, records of rainfall over several preceding months are one of the most important factors for predicting changes in DOC concentration on a catchment scale. Copyright © 2007 John Wiley & Sons, Ltd.

Contrasting nutrient exports from a forested and an agricultural catchment in south-eastern Australia

Dissolved organic carbon (DOC) and total and inorganic nitrogen and phosphorus concentrations were determined over 3 years in headwater streams draining two adjacent catchments. The catchments are currently under different land use; pasture/grazing vs plantation forestry. The objectives of the work were to quantify C and nutrient export from these landuses and elucidate the factors regulating export. In both catchments, stream water dissolved inorganic nutrient concentrations exhibited strong seasonal variations. Concentrations were highest during runoff events in late summer and autumn and rapidly declined as discharge increased during winter and spring. The annual variation of stream water N and P concentrations indicated that these nutrients accumulated in the catchments during dry summer periods and were flushed to the streams during autumn storm events. By contrast, stream water DOC concentrations did not exhibit seasonal variation.

Baseflow and stormflow metal concentrations in streams draining contaminated peat moorlands in the Peak District National Park (UK)

Leaching of previously deposited metals from atmospherically contaminated peat moorlands to receiving surface waters is an area of concern. Headwater streams in the Peak District National Park were sampled during baseflow and stormflow conditions to investigate the spatial and temporal variability in dissolved metal concentrations, the source of dissolved metals and the role of dissolved organic carbon (DOC) in the mobilisation and transport of dissolved metals. Under baseflow and stormflow conditions, Cu, Ni, Pb, Ti, V and Zn concentrations are highly variable. The results of this study reveal that Cu, Ni, Pb, V and Zn are leached from the contaminated peat soils into headwater streams. Ni and Zn are mobile within the peatland fluvial system due to poor sorption of these metals to organic matter. Elevated Zn concentrations in the headwater streams can be explained by the severely acidic nature of surface waters in this region. Stepwise multiple linear regression analysis reveals that the most important variable in explaining stormflow Pb, Ti and V concentrations is DOC. Due to the strong complexation of these metals by DOC, the export of dissolved Pb, Ti and V in peatland systems is likely to be controlled by DOC availability. Elevated stormflow dissolved Pb concentrations are due to the large store of Pb within the peat soils and high stream water DOC concentrations in surface waters of this upland area. Contemporary dissolved metal export from peat moorlands in the Peak District National Park may provide an analogue for future dissolved metal export in other contaminated peatland systems.

出水時における森林河川の溶存有機炭素濃度と斜面の水文過程

On the basis of hydrological processes in a well-characterized hillslope (0.65 ha), we analyzed the effects of flow paths on dissolved organic carbon (DOC) concentrations in a headwater stream (catchment area, 103 ha) during storm flow periods. In the hillslope, our previous findings of water flow paths and an analysis of the relationship between soil pore water pressure and hillslope discharge enabled us to classify the hillslope discharge representing water flow paths. During storm flow periods, the occurrence of upslope-derived water had the effect of keeping the stream DOC concentration low, reflecting that the DOC concentration of the upslope-derived water was very low. However, this effect was not observed during nonstorm flow periods, when the stream DOC concentration was nearly constant regardless of the occurrence of upslope-derived water. We inferred that the major reason that the effects of upslope-derived water on stream DOC concentration were different between storm and nonstorm flow periods was the difference in the timing of DOC export from a riparian area. We concluded that information on hydrological processes in hillslopes is helpful for elucidating the formation mechanism of stream DOC concentration in forested headwater catchments.

森林河川における溶存有機炭素濃度の縦断変化を支配する河畔湿地の個別性と分布パターン

Here, we discuss the effects of individuality and the distribution pattern of riparian wetlands on longitudinal changes in dissolved organic carbon (DOC) concentration in a headwater stream using model simulations. The model, which was composed of the catchment area, the flow distance of tributary channels, the DOC concentration of spring water and the DOC supply capacity of tributaries, well reproduced a longitudinal change in stream DOC concentration during a nonstorm period. This model calculation showed that the roles of riparian wetlands as DOC sources varied greatly among riparian wetlands, even though such wetlands were predominant sources of stream DOC. When we simulated for the longitudinal changes using three representative values (average, minimum and maximum) of the DOC supply capacity, the simulation results could not explain the features of the longitudinal changes, implying that the individuality of riparian wetlands strongly affects longitudinal changes. In addition, when we simulated the features under the different distribution patterns of riparian wetlands, i.e., wide, one-point and dispersed (two-point) distributions, there was a major difference between each simulation result, suggesting that the distribution pattern of riparian wetlands control longitudinal changes. Owing to the individuality and the distribution pattern of riparian wetlands, features of longitudinal changes in stream DOC concentration can vary largely even in the same region.

水の流出経路が森林河川の溶存有機炭素濃度に及ぼす影響

Here, we qualitatively discuss the effects of flow paths on dissolved organic carbon (DOC) concentrations in a headwater stream during a nonstorm flow (NF) period on the basis of hydrometric measurements at a hillslope and a riparian zone and the DOC concentrations of hillslope runoff water. In the analysis, the NF period in which dominant runoff water was derived from the upslope area in a hillslope (hillslope-NF period) was distinguished from the other NF period (normal-NF period). For hillslope runoff water, DOC concentrations during the hillslope-NF period were very low compared with those during the normal-NF period. For stream water, at a few measurement points, DOC concentrations during the hillslope-NF period were also very low (< 10th percentile of concentrations during normal-NF period), but at most measurement points the concentrations were slightly lower than those during the normal-NF period (at least 25th percentile). At sampling points with a relatively large catchment area (≥ 36 ha), the difference in DOC concentration between the hillslope- and normal-NF periods was small and nearly constant. It was inferred that stream DOC concentrations during NF periods were determined by the balance between the decreasing effects of hillslope water and the increasing effects of riparian water. Moreover, these results suggest that considering the balance is important for models estimating DOC concentrations in headwater streams.

河畔湿地表流水の硝酸態窒素と溶存有機炭素の濃度変化に対する表層土壌水の寄与

Riparian wetlands, which form the transition zone between hillslopes and streams, are important landscape units controlling water chemistry in catchments. Concerning the forest-related nonpoint source pollution in Japan, however, there have been few studies in which the roles of riparian wetlands in catchment water chemistry were clarified. We examined the changes in riparian stream nitrate (NO3-N) and dissolved organic carbon (DOC) concentrations in seven riparian wetlands during nonstorm periods in a forested headwater catchment. In addition, we discussed the contribution of surface soil waters to the changes in NO3-N and DOC concentrations. The stream NO3-N concentrations at the outlets of riparian wetlands were lower than those at the inlets; in contrast the stream DOC concentrations at the outlets were higher than those at the inlets. Among the riparian streams, the maximum changes in concentration per unit flow distance on a riparian wetland were 8- and 12-fold as high as the minimum changes in NO3-N and DOC concentrations, respectively. We conclude that for NO3-N, the differences in concentration change between riparian streams can result from the spatial variability of water permeability of surface soil; for DOC, the differences can be due to the spatial variability of both water permeability and soil water DOC concentration.

Connecting organic carbon in stream water and soils in a peatland catchment

We investigated the spatial and temporal connectivity between concentrations of organic carbon in stream water and the soil carbon pool in a small (1.3 km2) upland catchment in NE Scotland by comparing downstream changes in dissolved organic carbon (DOC) with spatial changes in the soil carbon pool. Stream water DOC concentrations increased downstream to reach a maximum in the center of the catchment and then decreased in the lower part of the catchment, a pattern that was linked to spatial changes in the soil carbon pool. Subcatchment analysis of the soil carbon pool and DOC export showed that the strong relationship between percent of peat coverage and stream water DOC in the upper part of the catchment did not hold farther downstream, where freely draining mineral soils become spatially more important. Temporal variations in stream water and soil water DOC concentrations in the central part of the catchment were studied using samples collected from two depths (0.20–0.45 m and 0.65–0.95 m) in the two most important soil types there (histosols and fluvisols). Stream water DOC concentrations were only related to soil water DOC in shallow peat. DOC concentration in shallow peat and stream water were both positively correlated with temperature. Multivariate regression analysis showed that 55% of the variation in stream water DOC concentrations could be explained by a model including stream temperature, discharge, and soil solution DOC concentrations in the shallow histosols. We conclude that the linkage between stream water DOC concentrations and the soil C pool in the upper 1.5 km of the stream reach are likely to be driven by temperature‐related DOC production in near‐surface histosols. Farther downstream, this relationship between organic carbon in the soil and stream becomes weaker as other processes (e.g., lower inputs from minerals soils and allochthonous within‐stream processing of DOC) become more important.

Environmental Controls of UV‐B Radiation in Forested Streams of Northern Michigan

We examined UV-B radiation flux and its environmental control within and among streams of northern Michigan. UV-B flux was estimated in streams by plastic dosimetry strips, which allow for the simultaneous and repeated in situ measurement of solar radiation. During the summer of 2004, UV-B flux was measured across depth gradients and along longitudinal transects in seven streams, which were chosen to encompass a range of dissolved organic carbon (DOC) concentrations and canopy cover. Attenuation coefficients of UV-B (K(d) (UV-B)) were estimated using plastic dosimeters placed along a depth gradient. K(d UV-B) were positively correlated with DOC concentration and similar to values obtained with laboratory and in situ spectrometry. Along 100 m longitudinal transects, UV-B flux varied along all streams regardless of their canopy cover and DOC concentration. Within-stream fluxes of UV-B were correlated to canopy cover in the only two streams that both had relatively low DOC concentration and variable canopy cover. Large differences were found among streams in the average UV-B flux (corrected for incident solar flux) reaching the dosimeters at 5 cm depth. These among-stream differences were largely accounted for by the stream width, canopy cover, and DOC concentration. Our results illustrate an inherent variability in UV-B flux within and among streams of northern Michigan that is strongly tied to the interactions of DOC concentration, stream size and riparian vegetation.

Nitrogen retention in the riparian zone of catchments underlain by discontinuous permafrost

Summary1. Riparian zones function as important ecotones that reduce nitrate concentration in groundwater and inputs into streams. In the boreal forest of interior Alaska, permafrost confines subsurface flow through the riparian zone to shallow organic horizons, where plant uptake of nitrate and denitrification are typically high.2. In this study, riparian zone nitrogen retention was examined in a high permafrost catchment (approximately 53% of land area underlain by permafrost) and a low permafrost catchment (approximately 3%). To estimate the contribution of the riparian zone to catchment nitrogen retention, we analysed groundwater chemistry using an end‐member mixing model.3. Stream nitrate concentration was over twofold greater in the low permafrost catchment than the high permafrost catchment. Riparian groundwater was not significantly different between catchments, averaging 13 μm overall. Nitrogen retention, measured using the end‐member mixing model, averaged 0.75 and 0.22 mmol N m−2 day−1 in low and high permafrost catchments, respectively, over the summer. The retention rate of nitrogen in the riparian zone was 10–15% of the export in stream flow.4. Our results indicate that the riparian zone functions as an important sink for groundwater nitrate and dissolved organic carbon (DOC). However, differences in stream nitrate and DOC concentrations between catchments cannot be explained by solute inputs from riparian groundwater to the stream and differences between streams are probably attributable to deeper groundwater inputs or flows from springs that bypass the riparian zone.

Amplified carbon release from vast West Siberian peatlands by 2100

Extensive new data from previously unstudied Siberian streams and rivers suggest that mobilization of currently frozen, high‐latitude soil carbon is likely over the next century in response to predicted Arctic warming. We present dissolved organic carbon (DOC) measurements from ninety‐six watersheds in West Siberia, a region that contains the world's largest stores of peat carbon, exports massive volumes of freshwater and DOC to the Arctic Ocean, and is warming faster than the Arctic as a whole. The sample sites span ∼106 km2 over a large climatic gradient (∼55–68°N), providing data on a much broader spatial scale than previous studies and for the first time explicitly examining stream DOC in permafrost peatland environments. Our results show that cold, permafrost‐influenced watersheds release little DOC to streams, regardless of the extent of peatland cover. However, we find considerably higher concentrations in warm, permafrost‐free watersheds, rising sharply as a function of peatland cover. The two regimes are demarcated by the position of the −2°C mean annual air temperature (MAAT) isotherm, which is also approximately coincident with the permafrost limit. Climate model simulations for the next century predict near‐doubling of West Siberian land surface areas with a MAAT warmer than −2°C, suggesting up to ∼700% increases in stream DOC concentrations and ∼2.7–4.3 Tg yr−1 (∼29–46%) increases in DOC flux to the Arctic Ocean.

Biogeochemical and hydrological controls on carbon export from a forested catchment in central Japan

AbstractHere we review research on the links between hydrological processes and the biogeochemical environment controlling the dynamics of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in temperate forested catchments. In addition, we present the results of original experiments. The spatial and temporal changes in DIC and DOC concentrations were investigated in tandem with observations of elementary belowground hydrological processes for a forested headwater catchment in central Japan. The soil CO2 gas concentration, which is the source of DIC, increased with depth. The hydrological characteristics of groundwater also affected the spatial variation of partial pressure of dissolved CO2 (pCO2) in groundwater. The temporal variations in the soil CO2 gas concentration and the pCO2 values of groundwater suggested that the dynamics of DIC were strongly affected by biological activity. However, the geographical differences in DIC leaching were affected not only by the link between climatological conditions and biological activity, but also by other factors such as geomorphologic conditions. The DOC concentrations decreased with selective removal of hydrophobic acid during vertical infiltration. The major DOC‐removal mechanisms were retention of metal‐organic complexes to soil solids in the upper mineral soil layer and decomposition of DOC in the lower mineral soil layer. The responses of the DIC and DOC concentrations to changes in discharge during storm events were explained by the spatial variation in the DIC and DOC concentrations. Seasonal variation, which represents a long‐term change, in stream water DOC concentrations was affected not only by the temporal variation in DOC concentrations in the topsoil, which may be affected by biological activity, but also by water movement, which transports DOC from the topsoil to stream water. These results indicate that both a biogeochemical approach and a method for evaluating the hydrological effects on carbon dynamics are critical for clarifying the carbon accumulation‐and‐release processes in forested ecosystems.

Stormflow Dynamics of Dissolved Organic Carbon and Total Dissolved Nitrogen in a Small Urban Watershed

We examined patterns of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) loading to a small urban stream during baseflow and stormflow. We hypothesized that lower DOC and TDN contributions from impervious surfaces would dilute natural hydrologic flowpath (i.e., riparian) contributions during storm events in an urban watershed, resulting in lower concentrations of DOC and TDN during storms. We tested these hypotheses in a small urban watershed in Portland, Oregon, over a 3-month period during the spring of 2003. We compared baseflow and stormflow chemistry using Mann–Whitney tests (significant at p<0.05). We also applied a mass balance to the stream to compare the relative significance of impervious surface contributions versus riparian contributions of DOC and TDN. Results showed a significant increase in stream DOC concentrations during stormflows (median baseflow DOC = 2.00 mg l−1 vs. median stormflow DOC = 3.46 mg l−1). TDN streamwater concentrations, however, significantly decreased with stormflow (median baseflow TDN = 0.75 mg l−1 vs. median stormflow TDN = 0.56 mg l−1). During storms, remnant riparian areas contributed 70–74% of DOC export and 38–35% of TDN export to the stream. The observed pattern of increased DOC concentrations during stormflows in this urban watershed was similar to patterns found in previous studies of forested watersheds. Results for TDN indicated that there were relatively high baseflow nitrogen concentrations in the lower watershed that may have partially masked the remnant riparian signal during stormflows. Remnant riparian areas were a major source of DOC and TDN to the stream during storms. These results suggest the importance of preserving near-stream riparian areas in cities to maintain ambient carbon and nitrogen source contributions to urban streams.

Role of discrete landscape units in controlling catchment dissolved organic carbon dynamics

The spatial sources and delivery mechanisms of dissolved organic carbon (DOC) to streams are poorly understood. We examined the relationship between storm DOC dynamics, catchment landscape units, and catchment scale to elucidate controls on DOC export dynamics at the Maimai watersheds, a group of highly responsive, steep, wet catchments located on the west coast of the South Island of New Zealand. Specifically, we address the controls on the characteristic hysteresis in DOC export dynamics (i.e., DOC concentrations higher on the rising than falling limb of the discharge hydrograph) previously ascribed to a flushing mechanism. We found that during the storm event, the proportion of riparian runoff was larger on the rising than falling limb of the hydrograph, while the proportion of hillslope runoff was smaller on the rising than falling limb of the hydrograph. The delayed response of hillslope runoff resulted in a disconnection between hillslope and riparian areas early in the event and higher DOC concentrations on the rising limb than the falling limb of the event hydrograph. Later in the event, hillslope and riparian areas became connected once the hillslope soil moisture deficits were satisfied. We suggest that the relative timing of riparian and hillslope source contributions and the connections and disconnections of dominant runoff contributing areas are the first‐order catchment controls on stream DOC concentrations and mass export.

Groundwater controls on nutrient cycling in a Mojave desert stream

1. Groundwater fluxes of nitrogen and dissolved organic carbon (DOC) were investigated in Grape Vine Canyon Stream in the Mojave Desert focusing on the rate of inputs and the fate of groundwater‐derived nutrients in the stream. Discharge rates from different ground waters were measured using an end‐member mixing model coupled with injections of a conservative solute tracer into the stream channel.2. In surface water, nitrate concentration averaged 1.13 mg N L–1 and DOC concentration averaged 1.82 mg C L–1.3. Groundwater discharge into Grape Vine Canyon Stream was derived from three sources. Nitrate concentration varied among the three groundwater sources with mean concentrations of 0.56, 0.94 and 0.08 mg N L–1. DOC, in contrast, did not vary among ground water sources, with an overall average concentration of 2.96 mg C L–1.4. In the surface stream, nitrate concentration was two‐fold greater than the concentration predicted from groundwater input, indicating that in‐stream processes generated nitrate. Stream DOC concentration was lower than predicted based upon groundwater input rate. The production of nitrate and loss of DOC suggest that DOC is lost through mineralisation of dissolved organic matter, possibly resulting in the mineralisation of dissolved organic nitrogen to ammonium and subsequent transformation to nitrate via nitrification. In further support of this hypothesised linkage, DOC loss explained 80–89% of the variance in nitrate production in Grape Vine Canyon Stream.

Factors regulating nitrification in aquatic sediments: effects of organic carbon, nitrogen availability, and pH

We investigated the response in nitrification to organic carbon (C) availability, the interactive effects of the C: nitrogen (N) ratio and organic N availability, and differing pH in sediments from several streams in the upper midwestern United States. In addition, we surveyed 36 streams to assess variability in sediment nitrification rates. Labile dissolved organic carbon (DOC) additions of 30 mg C·L–1 (as acetate) to stream sediments reduced nitrification rates (P &lt; 0.003), but lower concentration additions or dilution of ambient DOC concentration had no effect on nitrification. C:N and organic N availability strongly interacted to affect nitrification (P &lt; 0.0001), with N availability increasing nitrification most at lower C:N. Nitrification was also strongly influenced by pH (P &lt; 0.002), with maximum rates occurring at pH 7.5. A multiple regression model developed from the stream survey consisted of five variables (stream temperature, pH, conductivity, DOC concentration, and total extractable NH4+) and explained 60% of the variation observed in nitrification. Our results suggest that nitrification is regulated by several variables, with NH4+ availability and pH being the most important. Organic C is likely important at regulating nitrification only under high environmental C:N conditions and if most available C is relatively labile.

Biogeochemistry of a Treeline Watershed, Northwestern Alaska

Since 1950, mean annual temperatures in northwestern Alaska have increased. Change in forest floor and soil temperature or moisture could alter N mineralization rates, production of dissolved organic carbon (DOC) and organic nitrogen (DON), and their export to the aquatic ecosystem. In 1990, we began study of nutrient cycles in the 800-ha Asik watershed, located at treeline in the Noatak National Preserve, northwestern Alaska. This paper summarizes relationships between topographic aspect, soil temperature and moisture, inorganic and organic N pools, C pools, CO2 efflux, growing season net N mineralization rates, and stream water chemistry. Forest floor (O2) C/N ratios, C pools, temperature, and moisture were greater on south aspects. More rapid melt of the soil active layer (zone of annual freeze-thaw) and permafrost accounted for the higher moisture. The O2 C and N content were correlated with moisture, inorganic N pools, CO2 efflux, and inversely with temperature. Inorganic N pools were correlated with temperature and CO2 efflux. Net N mineralization rates were positive in early summer, and correlated with O2 moisture, temperature, and C and N pools. Net nitrification rates were inversely correlated with moisture, total C and N. The CO2 efflux increased with temperature and moisture, and was greater on south aspects. Stream ion concentrations declined and DOC increased with discharge. Stream inorganic nitrogen (DIN) output exceeded input by 70%. Alpine stream water nitrate (NO3-) and DOC concentrations indicated substantial contributions to the watershed DIN and DOC budgets.

Precambrian shield wetlands: Hydrologic control of the sources and export of dissolved organic matter

Most Precambrian Shield forested catchments have some wetland component. Even small riparian wetlands are important modifiers of stream chemistry. Dissolved organic matter (DOM) is one of the most important products exported by wetlands in streams. Stratigraphic control of hydraulic conductivity generally leads to decreasing conductivity with depth. Thus important flowpaths occur in the uppermost organic rich layers and are reflected in chemical profiles of dissolved organic carbon (DOC). Accumulation of DOC in peat porewaters is the net effect of production, consumption and transport. DOC profiles vary with degree of interaction with the surrounding upland catchment and distance from the edge of the wetland as well as internal processes within the wetland. In wetlands, DOM production is offset by flushing resulting in decreasing DOC concentrations with increasing flows. Despite old carbon (2,000 to 3,000 years) at relatively shallow depths, 14C activity in DOC exported from wetlands is mostly modern (recent carbon), consistent with shallow flowpaths and export of DOM from shallow organic rich horizons. In contrast, the source area for DOM in upland catchments with developed B horizon soils increases with antecedent soil moisture conditions resulting in increasing DOC concentrations with higher stream flows. Activity of 14C in stream DOC from upland catchments span a range from low activities (older carbon) similar to B horizon soil water during dry moisture conditions to values slightly less than modern (more recent carbon) during high moisture conditions. The more modern carbon activities reflect the increased contribution of the organic rich litter and A horizon soil layers in the area immediately bordering the stream under wet antecedent moisture conditions. Reduced hydrologic export or loss of wetlands under drier climatic conditions may result in in larger fluctuations in stream DOC concentrations and reduced DOM loads to lakes.

Colour - mass balances and colour - dissolved organic carbon relationships in lakes and streams in central Ontario

Colour and dissolved organic carbon (DOC) concentrations and mass balances were measured for seven unproductive lakes and 20 tributary streams between June 1980 and May 1992. Individual observations of colour, which measures an unknown DOC fraction assumed to be principally humic, were not good predictors of discrete DOC concentrations for any of the study sites. Long-term mean colour was, however, strongly correlated with long-term mean DOC concentrations in streams and lakes. Colour behaved differently from the total DOC pool in lakes. Lake retention and loss coefficients for colour were always higher than corresponding values for the total DOC pool, and the mean annual colour/DOC ratios in lake outflows were always less than the corresponding ratios in lake inputs, indicating that the coloured fraction was preferentially removed or photooxidized. A steady-state mass balance model was used to estimate the upper bound for the average coloured fraction of DOC in each lake. The upper bound for the seven lakes ranged from 34 to 88%, increasing with increasing colour, DOC, and total phosphorus.

Response characteristics of DOC flushing in an alpine catchment

The spatial distribution of source areas and associated residence times of water in the catchment are significant factors controlling the annual cycles of dissolved organic carbon (DOC) concentration in Deer Creek (Summit County, Colorado). During spring snowmelt (April–August 1992), stream DOC concentrations increased with the rising limb of the hydrograph, peaked before maximum discharge, then declined rapidly as melting continued. We investigated catchment sources of DOC to streamflow, measuring DOC in tension lysimeters, groundwater wells, snow and streamflow. Lysimeter data indicate that near-surface soil horizons are a primary contributor of DOC to streamflow during spring snowmelt. Concentrations of DOC in the lysimeters decrease rapidly during the melt period, supporting the hypothesis that hydrological flushing of catchment soils is the primary mechanism affecting the temporal variation of DOC in Deer Creek. Time constants of DOC flushing, characterizing the exponential decay of DOC concentration in the upper soil horizon, ranged from 10 to 30 days for the 10 lysimeter sites. Differences in the rate of flushing are influenced by topographical position, with near-stream riparian soils flushed more quickly than soils located further upslope. Variation in the amount of distribution of accumulated snow, and asynchronous melting of the snowpack across the landscape, staggered the onset of the spring flush throughout the catchment, prolonging the period of increased concentrations of DOC in the stream. Streamflow integrates the catchment-scale flushing responses, yielding a time constant associated with the recession of DOC in the stream channel (84 days) that is significantly longer than the time constants observed for particular locations in the upper soil. © 1997 John Wiley & Sons, Ltd.