Riparian hydraulic gradient and stream-groundwater exchange dynamics in steep headwater valleys

Patterns of riparian hydraulic gradients and stream-groundwater exchange in
headwater catchments provide the hydrologic context for important ecological processes.
Although the controls are relatively well understood, their dynamics during periods of
hydrologic change is not. We investigate riparian hydraulic gradients over three different
time scales in two steep, forested, headwater catchments in Oregon (WS01 and WS03) to
determine the potential controls of reach-scale valley slope and cross-sectional valley
geometry. Groundwater and stream stage data collected at high spatial and temporal
resolutions over a period encompassing a 1.25 year storm and subsequent seasonal
baseflow recession indicate that hydraulic gradients in both riparian aquifers exhibit strong
persistence of down-valley dominance. Responses to rainfall do not support the simple
conceptual models of increased riparian hydraulic gradient toward streams. Hydraulic
gradient response in WS01 to both the seasonal baseflow recession and the storm suggested
the potential for increased stream-groundwater exchange, but there was less evidence for
this in WS03. Results from four constant-rate tracer injections in each stream showed a
high baseline level of exchange overall, and both a slight seasonal increase (WS01) and
slight decrease (WS03) in the riparian intrusion of tracer-labeled stream water as stream
discharge receded. These results indicate that steep headwater valley floors host extensive
stream water exchange and very little change in the water table gradients over 3 orders of
magnitude of stream discharge.