DISCERNING PATTERNS OF FLUVIAL EROSION AND ALLUVIATION USING THE STREAM GRADIENT INDEX AND SHORT-LIVED RADIONUCLIDES

Approximately 50 years ago, J.T. Hack introduced the concept of the stream gradient index (SGI; Hack, 1957), defined as the product of local stream channel slope and downstream distance from headwaters. Since then, numerous studies have employed the SGI as a useful indicator of large-scale disequilibrium processes, such as active faulting, occurring along stream profiles. A theoretical framework indicates that the SGI serves as a simple proxy for stream power; thus the overall magnitude of the SGI (in the case of strictly supply-limited channels) or its first derivative (mixed, transport-limited channels) may be indicative of channel incision or alluviation. To explore this potential, we analyze along-stream concentrations of short-lived radionuclides 7Be and 210Pb in channel sediments in the context of along-stream values of the SGI and other sediment transport parameters (such as the channel Shields number) in two different settings: Mink Brook in the glaciated terrain and temperate climate of Grafton County, NH and Olema Creek in the Mediterranean climate and tectonic activity of Marin County, CA.