Linking critical zone currencies to states of river ecosystems
In 1975, Hynes wrote“...in every respect the valley rules the stream”. The valley’s vegetation, soils and topography determine the stream’s hydrograph and inputs of solutes, sediments and organic matter, which in turn govern the structure and function of stream food webs. Hynes’ vision of the landscape context of streams anticipated Critical Zone science, which explores processes linking the top of the vegetative canopy to the top of fresh bedrock at the bottom of the water table. Since Hynes wrote, we have learned that streams are not just slaves to their valleys, but exert reciprocal influence, on landscapes through evolution driven by channel incision, and on ecosystem through biological backflows. Rempe and Dietrich (2014) propose that the runoff that sustains rivers during drought is generated from storage between the landscape surface (Zo) and the bottom of the critical zone (Zb), where a weathering front meets underlying fresh bedrock of much lower conductivity. Water stored between these surfaces is tapped by vegetation or released as runoff to rivers or other wet habitats. Uplift and channel incision allow bedrock to drain and weather, so control the thickness of this water-yielding stratum. Material rock properties affect porosity and conductivity, so along with surface (Zo) and subsurface (Zb) topography, control residence times and release rates of water. This new bottom-up view of hillslope hydrology adds rock moisture to soil as the reservoir where water is stored, transformed, and released as runoff. Critical zone processes may explain how topography, geology, and vegetation will govern the release of water, heat, sediments, solutes, organic matter, and organisms to rivers in ways that sustain, heal or damage them.
The Eel River of northwestern California flows through a region that is considered relatively well watered, but is increasingly stressed by drought and summer water withdrawals for marijuana cultivation. Without sufficient summer baseflow, the Eel may tip from a recovering salmon-supporting ecosystem towards a cyanobacterially degraded one. To maintain food webs and habitats that support salmonids and suppress harmful cyanobacteria, the river needs enough summer discharge to cool, gently flush and hydrologically connect channel habitats with base flow. Rearing salmon and steelhead can survive even in pools that become isolated during summer low flows if hyporheic exchange is sufficient. But if summer baseflows are too low, warm stagnant conditions stress salmonids physiologically, and degrade food webs by allowing potentially toxic cyanobacteria to overgrow edible algae like diatoms.