Abstract. Changes in the steepness of river profiles or abrupt vertical steps (i.e. waterfalls) are thought to be indicative of changes in erosion rates, lithology or other factors that affect landscape evolution. These changes are referred to as knickpoints or knickzones and are pervasive in bedrock river systems. Such features are thought to reveal information about landscape evolution and patterns of erosion, and therefore their locations are often reported in the geomorphic literature. It is imperative that studies reporting knickpoints and knickzones use a reproducible method of quantifying their locations, as their number and spatial distribution play an important role in interpreting tectonically active landscapes. In this contribution we introduce a reproducible knickpoint and knickzone extraction algorithm that uses river profiles transformed by integrating drainage area along channel length (the so-called integral or χ method). The profile is then statistically segmented and the differing slopes and step changes in the elevations of these segments are used to identify knickpoints, knickzones and their relative magnitudes. The output locations of identified knickpoints and knickzones compare favourably with human mapping: we test the method on Santa Cruz Island, CA, using previously reported knickzones and also test the method against a new dataset from the Quadrilátero Ferrífero in Brazil. The algorithm allows for the extraction of varying knickpoint morphologies, including stepped, positive slope-break (concave upward) and negative slope-break knickpoints. We identify parameters that most affect the resulting knickpoint and knickzone locations and provide guidance for both usage and outputs of the method to produce reproducible knickpoint datasets.

Introduction I Sample Acquisition and Preparation 2 Characterization of Starting Materials 3 Experimental Techniques 8 Results of Aqueous Phase Analyses 250°C Experiments 150°C Experiments 90°C Experiments Results of Solid Phase Analyses Discussion 21 Acknowledgments Bibliography 26

The Nevada Nuclear Waste Storage Investigations Project has conducted experi ments to study the hydrothermal interaction of rock and water representative of a poten tial high-level waste repository at Yucca Mountain, Nevada. The results of these experi ments help define the near-field repository environment during and shortly after the thermal period that results from the emplacement of nuclear waste. When considered in conjunction with results contained in companion reports, these results cm be used to assess our ability to accelerate tests using the surface area/volume parameter and/or temperature. These rock-water interaction experiments were conducted with solid polished wa fers cut from both drillcore and outcrop samples of Topopah tuff, using both a natural ground water and distilled water as the reacting fluid. Pre-and post-test characterization of the reacting materials was extensive. Post-test identification and chemical analysis of secondary phases resulting from the hvdrothermal interactions were aided by using monoliths of tuff rather than crushed material. All experiments were run in Dicksontvpe, gold-bag rocking autoclaves that were periodically sampled at in situ conditions. A total of nine short-term (up to 66-day) experiments were run in this series; these experi ments covered the range from 90 to 250°C and from 50 to 100 bar. The results obtained from the experiments have been used to evaluate the modeled results produced by cal culations using the geochemieal reaction process code EQ3/6.

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Abstract. Many mountain ranges survive in a phase of erosional decay for millions of years following the cessation of tectonic activity. Landscape dynamics in these post-orogenic settings have long puzzled geologists due to the expectation that topographic relief should decline with time. Our understanding of how denudation rates, crustal dynamics, bedrock erodibility, climate, and mantle-driven processes interact to dictate the persistence of relief in the absence of ongoing tectonics is incomplete. Here we explore how lateral variations in rock type, ranging from resistant quartzites to less resistant schists and phyllites, and up to the least resistant gneisses and granitic rocks, have affected rates and patterns of denudation and topographic forms in a humid subtropical, high-relief post-orogenic landscape in Brazil where active tectonics ended hundreds of millions of years ago. We show that catchment-averaged denudation rates are negatively correlated with mean values of topographic relief, channel steepness and modern precipitation rates. Denudation instead correlates with inferred bedrock strength, with resistant rocks denuding more slowly relative to more erodible rock units, and the efficiency of fluvial erosion varies primarily due to these bedrock differences. Variations in erodibility continue to drive contrasts in rates of denudation in a tectonically inactive landscape evolving for hundreds of millions of years, suggesting that equilibrium is not a natural attractor state and that relief continues to grow through time. Over the long timescales of post-orogenic development, exposure at the surface of rock types with differential erodibility can become a dominant control on landscape dynamics by producing spatial variations in geomorphic processes and rates, promoting the survival of relief and determining spatial differences in erosional response timescales long after cessation of mountain building.