Does Moss Grow on A Rolling Stone? Assessing the Influence of Hydrologic Regime on Bed Load Transport and In-Stream Primary Producers in Steep Mountain Channels

Conceptual Setup
It has long been realized that earth systems are open, interconnected, and exhibit complex and emergent patterns as a result of natural forcings. As an example of such interactions, consider the large number of biotic and abiotic factors that influence the quality and availability of in-channel habitat. Stream channels are much like conveyor belts; they transport biotic and abiotic material from the steep uplands of a watershed to the lower-elevation, lower-slope depositional portions of a landscape. The channel at any point in the network responds to local inputs from hillslopes and tributaries and to material transferred from upstream. These are some of the physical processes that affect stream habitat generation; other factors influencing habitat quality are dominantly driven by biotic factors such as species competition, species diversity and abundance, and availability of in-stream food resources.

One of the challenges faced by scientists trying to understand what environmental factors act as dominant controls on habitat selection or species diversity is the large number of variables that must be measured or accounted for. In the example of habitat generation discussed above, there would be several physical processes an investigator would need to understand: the hydrologic regime, that is, the magnitude, duration, frequency and intensity of hydrologic events, the magnitude and frequency of bedload transport, and the rate and character of material inputs from upstream reaches. In addition, biotic factors such as species diversity, abundance, and interactions would need to be quantified to understand the dynamic linkages present within the system. In this vignette I will provide a brief example of how to experimentally approach complex, open and interconnected systems and provide some initial results and interpretations.

Case Study Example
Increases in air surface temperatures and changes in rain-snow proportions will likely result in complex alterations of hydrologic regimes throughout high-relief topography (IPCC, 2007). Changes in the magnitude, frequency, intensity, and duration of hydrologic events could, in turn, alter the current sediment transport regimes of tributary channels. These anticipated changes promote the need to understand how changes in hydrograph form will affect sediment transport regimes and in-stream primary producers in steep mountain channels (Figure 1). The primary goals of this pilot study are to quantify the current hydrologic and sediment transport regimes of steep mountain channels across a rain-to-snow gradient, understand how the physical disturbance regime influences aquatic primary producers, and make predictions of how climate change could affect these processes.

An innovative way to generate these observations utilizes a space-for-time approach where study sites at different geographic locations are carefully chosen to offer an analogue for the potential temporal evolution of both biotic and abiotic processes. This approach was carried out in the Salmon River watershed, central Idaho, United States, where runoff, bedload transport distances and periphyton were monitored for 2 water years for 12 channels equally sub-grouped into rain-dominated (RD), mixed rain and snow (MRS) and snow-dominated (SD) categories (Figure 2). RD watersheds provide insights into the potential biotic and abiotic changes that MRS and SD channels could experience if climate warming occurs.

Observations of streamflow from the different precipitation regimes indicate that differences in precipitation phase and amount generate significant differences in the timing, frequency, magnitude and duration of hydrologic events (Figure 3). RD systems were characterized by spiky hydrograph forms, with quick rise and fall from baseflow during flood events. Peak runoff in RD channels tended to be lower in magnitude than MRS or SD systems and events were shorter in duration; however, hydrologic flow events were more frequent and occurred throughout winter, spring, and early summer. RD channels experienced the greatest variability in streamflow conditions. SD channels spent most of the year at baseflow conditions and transported the bulk of streamflow during snowmelt in the late spring and early summer. Flows tended to increase rapidly and stay high in SD channels once synoptic warming occurred. MRS watersheds were also characterized by high-magnitude, long-duration, melt-driven flood events but exhibited higher rates of increase and decrease in streamflow because of frequent rain-on-snow events. MRS channels became active earlier in the year than SD ones because of earlier warming and late season precipitation falling as rain instead of snow.

Observations of high-magnitude, long-duration hydrologic events in SD channels might lead one to expect greater amounts of sediment transport in these systems. However, results from tracking stream-bed cobbles tagged with Passive-Integrative-Transponders revealed that travel distances were much greater in RD watersheds than in MRS or SD watersheds (Figure 4). This seemingly incongruent result between flood events and sediment transport distances in RD and MRS and SD watersheds likely reflects differences in the ability of the supplied water to erode bed sediments. It is important to note that SD and MRS channels had higher degrees of armoring than RD channels. This likely indicates that MRS and SD channels experience transport-capable floods on an annual basis and that the floods are of great enough temporal duration to transport any newly supplied sediment; what remains in the channel is the un-transportable sediments. In contrast, the lower frequency and shorter duration of sediment-transportable flows in RD channels are not as efficient at armoring the channel and generating mature bedforms (an idea supported by the work of Hassan, et al, 2006). Thus, although the transport capability of SD and MRS channels is greater than RD ones, the higher degree of armoring and grain-packing in SD and MRS channels generates more erosion-resistant channels and creates the "sediment transport paradox".

Combining streamflow and sediment transport analyses with periphyton (a mixture of algae and cyannobacteria that is an important in-stream food resource) analyses indicates that greater variability in streamflow conditions and greater sediment transport distances resulted in higher variability in periphyton mass within and between channels in RD portions of the landscape. While these are only initial results and need further validation, they suggest that RD watersheds have a higher probability of stochastic or randomly generated flood events, greater variability in overall streamflow conditions, and less streambed armoring, resulting in greater sediment transport distances. In turn, this highly variable and stochastically driven disturbance regime generates higher variability in streambed periphyton development.

While this study is still in initial stages, it represents important progress in linking paired biotic and abiotic systems, quantifying the diversity of hydrologic, sediment transport, and ecological conditions possible in high relief topography, and offers new insights into strategies for managing watersheds in the context of a changing climate.

• Hassan, M. A., R. Egozi & G. Parker (2006) Experiments on the effect of hydrograph characteristics on vertical grain sorting in gravel bed rivers. Water Resources Research, 42.
• IPCC, 2007, Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change: Core Writing Team, Pachauri, R.K. and Reisinger, A. (Eds.), IPCC, Geneva, Switzerland, 104 p.