Coevolution of hydrology and topography on a basalt landscape in the Oregon Cascade Range, USA

Abstract

Young basalt terrains offer an exceptional opportunity to study landscape and hydrologic evolution through time, since the age of the landscape itself can be determined by dating lava fl ows. These constructional terrains are also highly permeable, allowing one to examine timescales and process of geomorphic evolution as they relate to the partitioning of hydrologic fl owpaths between surface and sub-surface fl ow. The western slopes of the Cascade Range in Oregon, USA are composed of a thick sequence of lava fl ows ranging from Holocene to Oligocene in age, and the landscape receives abundant precipitation of between 2000 and 3500 mm per year. On Holocene and late Pleistocene lava landscapes, groundwater systems transmit most of the recharge to large springs (≥0·85 m 3 s −1) with very steady hydrographs. In watersheds >1 million years old, springs are absent, and well-developed drainage networks fed by shallow subsurface stormfl ow produce fl ashy hydrographs. Drainage density slowly increases with time in this basalt landscape, requiring a million years to double in density. Progressive hillslope steepening and fl uvial incision also occur on this timescale. Springs and groundwater-fed streams transport little sediment and hence are largely ineffective in incising river valleys, so fl uvial landscape dissection appears to occur only after springs are replaced by shallow subsurface stormfl ow as the dominant streamfl ow generation mechanism. It is proposed that landscape evolution in basalt terrains is constrained by the time required for permeability to be reduced suffi ciently for surface fl ow to replace groundwater fl ow.

Chemical weathering fluxes from volcanic islands and the importance of groundwater: The Hawaiian example

“…The downstream abrupt increase in silica below the major nick points in the Kohala and Waimea canyons indicates that groundwater is an important contributor to stream discharge in deeply incised valleys. Jefferson et al (2010) investigated the drainage development of a basaltic chronosequence in the Oregon Cascades, paying special attention to the partitioning of water among different flowpaths and how this partitioning evolves over time. There are some clear parallels between their observations and ours, although a direct comparison is not straightforward because of a lack of comparable metrics in the two studies - Jefferson et al (2010) use hydrographs as a proxy for flowpaths whereas in this study, element concentrations are used to infer about flowpaths.…”

“…Jefferson et al (2010) investigated the drainage development of a basaltic chronosequence in the Oregon Cascades, paying special attention to the partitioning of water among different flowpaths and how this partitioning evolves over time. There are some clear parallels between their observations and ours, although a direct comparison is not straightforward because of a lack of comparable metrics in the two studies - Jefferson et al (2010) use hydrographs as a proxy for flowpaths whereas in this study, element concentrations are used to infer about flowpaths. Jefferson and her co-workers (2010) found drainage density (km/km 2 ) to be an excellent indicator of drainage development, showing a clear increase with age in their studied watersheds.…”

“…There is no permanent surface drainage or welldeveloped drainage network on the young Kilauea or Mauna Loa substrates, although a few channels are active during heavy precipitation events. In the Oregon Cascades young volcanic surfaces are drained by spring-fed streams whose discharge is relatively insensitive to rainfall events (Jefferson et al, 2010). In the Cascades, the hydrograph becomes flashier as streams age, the drainage network develops and surface flow takes over from baseflow as the main contributor to stream discharge.…”

“…As a consequence, flowpaths of water change (Lohse and Dietrich, 2005;Jefferson et al, 2010). On young, unweathered surfaces most rainfall percolates uninterrupted to the groundwater table, and streams do not occur on the surface.…”

Parameterizing sub-surface drainage with geology to improve modeling streamflow responses to climate in data limited environments

“…In most hydrologic modeling studies, parameters associated with effective conductivity, such as hydraulic conductivity and macropore distributions, are calibrated or assumed to be spatially uniform. Given that subsurface drainage properties evolve through landscape evolutionary processes, one might expect that these parameters would vary across geological classification (Jefferson et al, 2006(Jefferson et al, , 2010. Empirical studies and models based on streamflow patterns in the Oregon Cascades support this assertion (Tague andGrant, 2004, 2009).…”