Storm flow in forested basins on the Canadian Shield is largely supplied by subsurface water; however, mechanisms by which this water reaches the stream remain unclear. Side slope contributions to storm flow were studied using throughflow trenches on slopes in a headwater basin near Dorset, Ontario. Discharge, soil water content, and chemical and isotopic signatures of subsurface water were monitored at each site. Four hypotheses were tested: (1) most flow occurs at the soil-bedrock interface on shield slopes with thin soil; (2) a significant fraction of event water moves vertically to bedrock via preferential flow pathways and laterally over the bedrock surface; (3) relative preevent water contribution to subsurface flow on shield slopes is a function of soil thickness; and (4) a significant portion of event water flux in storm flow from forested basins with shallow soil cover is supplied from side slopes via subsurface flow along the soil-bedrock interface. Hypothesis 1 was confirmed from hydrometric observations during spring and fall rainstorms. Hypotheses 2 and 3 were supported by temporal trends in dissolved organic carbon and •80 in flow at the soil-bedrock interface and by isotopic hydrograph separations (IHSs) of hillslope runoff. Comparison with the streamflow IHS indicated that event water flux from the basin in excess of that attributable to direct precipitation onto near-channel saturated areas could be supplied by flow along the bedrock surface (hypothesis 4). Flow at the soil-bedrock interface on side slopes also contributed ---25% of preevent water flux from the basin. Much of the event water component of basin storm flow may travel considerable distances via subsurface routes and is not necessarily contributed by surface runoff processes (Horton flow or saturation overland flow). Therefore the assumption that event water undergoes little interaction with the soil during its passage downslope may be unwarranted here. One concept which may account for rapid subsurface transfer of preevent water to stream channels is the groundwater ridging mechanism [Sklash and Farvolden, 1979]. This process is limited to the near-channel area and results from rapid conversion of the tension-saturated zone (capillary fringe) tophreatic water by infiltrating event water. The subsequent rise in the near-stream water table increases the hydraulic gradient to the stream and/or the size of seepage faces, resulting in pronounced groundwater discharge to the channel. This rapid Copyright 1995 by the American Geophysical Union. Paper number 94WR03286. 0043-1397/95/94WR-03286505.00 rise in the water table has been noted in laboratory [Abdul and Gillham, 1984] and field investigations [Novakowski and Gillham, 1988; Abdul and Gillham, 1989] and is supported by numerical simulation [SMash and Farvolden, 1979]. However, studies on the Canadian Shield have questioned the ability of groundwater ridging to serve as the dominant means of supplying storm flow to the stream during snowmelt. Thus Buttle and Sami [1992] found that gro...

Methodological issues associated with isotopic hydrograph separations (IHSs) in built-up environments are explored using results from the 1990 spring melt in a suburban basin in Peterborough, Ontario, Canada. The heterogeneous nature of suburban environments complicates the selection of appropriate isotopic signatures for event and pre-event waters. Near-stream groundwater 6l8O sampled from wells was poorly mixed, such that the pre-event water signature was best characterized by 6l8O in pre-melt baseflow or discharge from a headwater spring. The event water signature during snowmelt can be characterized using 6l80 in the pre-melt snowpack, surface runoff samples or meltwater from lysimeters. However, the use of snowpack 6 l 8 0 may be inappropriate in suburban basins where meltwater from thin snowcover may exhibit pronounced res onses to 6 l 8 0 in rainfall contributions. Intensive sampling of the spatial variability of runoff or meltwater 6' 0 may be required to characterize the average event water signature adequately. Rainfall 6l8O provided an appropriate event water signal during a large rain on snow event, and differences between this IHS and one generated using an event water signature that included meltwater contributions from snow-covered surfaces were within the uncertainty attributable to the analytical error in 6l80 values. Event water supplied 55-63% of the peak discharge and 48-58% of total runoff from the basin during the melt, which is consistent with the fraction of the basin that has been developed. These results contrast with IHSs conducted in forested basins that suggest that stormflow is dominated by pre-event water contributions.

Current conceptual runoff models hypothesize that stormflow generation on the Canadian Shield is a combination of subsurface stormflow and saturation overland flow. This concept was tested during spring runoff in a small (3.3 ha) headwater basin using: (1) isotopic and chemical hydrograph separation and (2) field mapping and direct tracing of saturated areas. Isotopic and chemical hydrograph separation indicated three runoff components: (1) pre-melt subsurface flow; (2) subsurface flow of new (event) water; and (3) direct precipitation on to saturated areas (DPS). During early thaw-freeze cycles, their relative contributions to total flow remained constant (65 per cent, 30 per cent, and 5 per cent respectively). It is hypothesized that lateral flow along the bedrock/mineral soil interface, possibly through macropores, supplied large volumes of subsurface flow (of both old and new water) rapidly to the stream channel. Much higher contributions of DPS were observed during an intensive rain-on-snow event (15 per cent of total flow). Mapping and direct tracing of saturated areas using lithium bromide, suggested that saturated area size was positively correlated to stream discharge but its response lagged behind that of discharge. These observations suggest that the runoff mechanisms, and hence the sources of stream flow, will vary depending on storm characteristics.

No abstract

This report describes work in an IHD Representative Basin in the Quebec Appalachians, the Eaton River Basin (86 km2 in area), upstream from Randboro. The Basin is dominantly forest-covered, contains no large settlement, and, in general, shows little human disturbance that might affect sediment production. The suspended load of the Eaton River was studied in detail during the spring runoff periods of 1970 and 1971; available long-term discharge data indicate these to be representative of present-day conditions. Sediment transport rates are well below capacity and sediment yields are lower than might have been expected from the Langbein-Schumm data in the United States. Suspended sediment originates primarily from scour of the banks of the channel network, and concentrations show a systematic increase with basin area (or distance downstream), quite unlike previous data from the midwestern United States. The sediment rating curve approach is a very good predictor of sediment transport rates, although because of the differences in hydrograph type, there is a large difference between the equations for the 1970 and 1971 spring floods. This difference, and residuals from the sediment rating curves, are considered in a simulation model of sediment production from bank erosion based on the changing shear resistance of bank sediment during a fluctuating hydrograph.Ce rapport dkrit le travail effectut dans un bassin representatif (IHD) des Appalaches quebkoises, le bassin de la riviece Eaton (86 km2 de superlicie) en amot de Randboro. Le bassin est en grande partie boisC, est peu peuple et, en general, semble peu affect6 par I'homme dans sa production de sediments. La charge en suspension de la riviere Eaton a ett Ctudik en dCtail durant les crues printanieres de 1970 et 1971 ; les donnks disponibles de la dkcharge a long terme indiquent qu'elles sont representatives des conditions actuelles. Les taux de transport des sediments sont de beaucoup inferieurs a la capacite et la production de sediments est inferieure a ce a quoi on aurait pu s'attendre a partir des donnks de Langbein-Schumm. Les matieres en suspension ont leur origine principalement de IVrosion des rives du reseau et les concentrations indiquent une augmentation systematique proportionnelle a la surface du bassin (ou a la distance en aval) ce qui est trks different des donndes connues du midwest americain. La methode de pr6dire le taux de transport des sediments par une wurbe s'avee tres bonne m&me si, a cause de differences dans le type hydrographique, il y a une grande difference entre les equations des crues printanieres de 1970 et 1971. On tient compte de cette difference, et autres details des courbes de taux de sedimentation, dans un modele simulant la production de sediments a partir de 1'Crosion des rives lorsqu'on modifie la resistance au cisaillement de sediments du rivage au cours de fluctuations hydrographiques.[Traduit par le journal]