Thresholds of strath genesis deduced from landscape response to stream piracy by Pancho Rico Creek in the Coast Ranges of central California

Abstract

Recent, compelling arguments regarding the overwhelming influence of drainage basin lithology on strath formation (Montgomery, 2004) require reevaluation of the role other factors play in this process. Pancho Rico Creek ("PRC") is a bedrockeroding stream in the tectonically active central Coast Ranges of California. Late-Quaternary time stream piracy of the upper San Lorenzo Creek catchment by PRC caused 'instantaneous' (at graded timescales) catchment-area expansion (from 99.5 - 156.6 km2) and a significant change in the lithologic composition of PRC channel bedload. This naturally occurring "experiment" in the southern Gabilan Range provides insight regarding the influences of catchment area and channel-bedload composition on formation of fluvial straths in the context of newly recognized controls on bedrock-channel erosion (Sklar and Dietrich, 2001; Stock and others, 2005). Distinct compositional differences in pre- and post-capture PRC alluvium facilitates deducing from alluvial stratigraphy pre- and post-capture fluvial processes. The pre-capture drainage basin of PRC, known as Pancho Rico Valley, is entirely underlain by predominantly fine-grained, friable, clastic marine sediments of the Pancho Rico Formation. Pre-capture PRC alluvium consists only of Pancho Rico Formation clasts, but post-capture PRC alluvium also includes relatively competent clasts of Franciscan Complex rocks. Pre- and post-capture straths formed in Pancho Rico Valley only along reaches of PRC where drainage area was/is greater than ca. 60 km2. This indicates that a minimum catchment area is necessary for PRC to carve straths. Spatial and temporal patterns of strath formation in Pancho Rico Valley support the hypothesis that distribution of straths within a drainage basin underlain by mechanically weak rock is largely dependent on catchment area. Two hypotheses are proposed to explain differences in the charact er of pre- and post-capture straths as well as the processes leading to their formation and/or preservation. Pre-capture straths are paired, major straths, which indicate that lateral incision prevailed over vertical incision during a given interval of graded time (for example, Bull, 1990). Post-capture straths are unpaired, minor straths, which indicate simultaneous vertical and lateral incision during a given interval of graded time (for example, Bull, 1990). One hypothesis proposes that PRC was able to simultaneously (over graded timescales) lower and widen its channel in post-capture time because the presence of relatively competent bedload enhanced abrasion and accelerated channel denudation. Relatively incompetent bedload precluded simultaneous channel lowering and widening in pre-capture time. An alternative, favored hypothesis is that post-capture minor straths are present along PRC because they are buried under Franciscan-Complex derived gravelly alluvium, which is more resistant to weathering than Pancho-Rico-Formation derived gravelly alluvium. Thin deposits typical of minor erosional straths, and composed of readily weathered Pancho Rico Formation bedload, were insufficient to protect straths from weathering and erosion in pre-capture time. Pre-capture straths survived only when buried by substantial amounts of alluvium, as may have occurred during an episode of strath formation and alluviation caused by climate change.