The Bolivian Andes flank one of Earth's major topographic features and dominate sediment input into the Amazon Basin. Millennial-scale erosion rates and dominant controls on erosion patterns in this range are poorly known. To define these patterns, we present 48 erosion rate estimates, derived from analysis of in situ 10 Be in quartz-bearing alluvium collected from the Upper Beni River basin.Erosion rates, corrected for the non-uniform distribution of quartz in the sample basins, range from 0·04 mm a ). Hence, our data do not record any significant variation in erosion rate over the last several million years. Mean and modal short-term erosion rates for the Andes are an order of magnitude lower than rates in the Ganges River headwaters in the High Himalaya and an order of magnitude greater than rates typical of the European Alps.In the Upper Beni River region of the Bolivian Andes, short-term, basin-averaged erosion rates correlate with normalized channel steepness index, a metric of relative channel gradient corrected for drainage area. Neither normalized channel steepness index nor basin-averaged erosion rate shows strong correlation with mean basin hillslope gradient or mean basin local relief because many hillslopes in the Upper Beni River region are at threshold values of slope and local relief. Patterns of normalized channel steepness index appear primarily to reflect tectonic patterns and transient adjustment to those patterns by channel networks. Climate and lithology do not appear to exert first-order controls on patterns of basin-averaged erosion rates in the Bolivian Andes.

Thirty to forty m.y. of post-Laramide degradation of the southern Rocky Mountains likely produced relatively low-relief topography within the crystalline cores of the ranges, and capped the adjacent sedimentary basins with easily eroded sediments. We focus on the modern, more dissected topography of these ranges, refl ecting late Cenozoic evolution driven by fl uvial and glacial exhumation, each of which affects different portions of the landscape in characteristic ways. Ongoing exhumation of the adjacent basins, in places by more than 1 km, is effectively lowering base level of streams draining the crystalline range cores. The streams have incised deep bedrock canyons that now cut the fl anks of the range. Over the same time scales, glaciation of the headwaters of the major streams has modifi ed the range crests. We utilize the topography of the northern Front Range of Colorado to explore the response of a Laramide range both to the exhumation of the adjacent basin and to glaciation in the high elevations. We break the problem of whole landscape evolution into three related, one-dimensional problems: evolution of the high smooth summit surfaces; evolution of the longitudinal profi les of adjacent glacial troughs; and evolution of the fl uvial profi les downstream of the glacial limit. We review work on the high summit surfaces, showing quantitatively that they are steady-state features lowering at rates on the order of 5 μm/yr, and are entirely decoupled from the adjacent glacial troughs. Glaciers not only truncate these high surfaces, but greatly alter the longitudinal profi les of the major streams: major steps occur at tributary junctions, and profi les above the glacial limit are signifi cantly fl attened from their original fl uvial slopes. We extend existing models of glacial valley evolution by including processes that allow headwall retreat. This serves to enhance the headward retreat of east-facing valleys, and explains the asymmetric truncation of the high smooth surfaces that form the spine of the range. Fluvial profi les downstream of the glacial limit commonly display a prominent convexity inboard of the range edge. Stream-power-based numerical models of profi le evolution of specifi c rivers demonstrate that this refl ects a transient response of the streams to base-level lowering. This response varies signifi cantly with drainage basin area. We explore the degree to which this differential response controls the R.S. Anderson et al. location of major remnants of pediments on the edge of the Great Plains, such as the prominent Rocky Flats and adjacent surfaces.

The Cordillera Real of the Bolivian Andes is a large, tectonically active mountain range that dominates sediment influx into the Amazon Basin, but rates of exhumation in the orogen are poorly known. We present 20 new apatite fission track ages from two valleys in the Cordillera Real to constrain patterns of mountain range exhumation over 10 6-10 7 yr. We interpret these and previously published data from a third valley using a 2-D thermal model that accounts for topographic and advective influences on measured cooling ages. Exhumation rates in the Cordillera Real are ∼0.2-0.6 mm/yr, comparable to rates in parts of Denali, the Washington Cascades, the Olympic Mountains, and the European Alps and an order of magnitude slower than rates in Taiwan, Nanga Parbat, the Greater Himalaya of Nepal, and the Southern Alps of New Zealand. Three-to fourfold cooling age variations in the Cordillera Real imply at least twofold exhumation rate variations within and between valleys over distances of only tens of kilometers. Topography in the cross-valley dimension affects exhumation rate estimates by 20%-30% in the downstream portions of two sample transects. Along-valley topographic effects are less significant in this setting, affecting exhumation rate estimates by !15%. The most significant along-valley topographic effects are associated with long-wavelength mountain shape, including both retreat of the closure temperature isotherm near the mountain crest and compression of low-temperature isotherms farther down the mountain flank. Locally varying phenomena (e.g., subregional structural history or transient patterns of local channel incision) must exert important controls on long-term erosion patterns in order to produce observed short-wavelength exhumation rate variations. Comparison of exhumation rate estimates with modern erosion rates suggests that long-term and short-term average erosion rates likely vary by less than twofold.

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Rivers carved into uplifted plateaus are commonly disrupted by discrete events from the surrounding landscape, such as lava fl ows or large mass movements. These disruptions are independent of slope, basin area, or channel discharge, and can dominate aspects of valley morphology and channel behavior for many kilometers. We document and assess the effects of one type of disruptive event, lava dams, on river valley morphology and incision rates at a variety of time scales, using examples from the Owyhee River in southeastern Oregon.Six sets of basaltic lava fl ows entered and dammed the river canyon during two periods in the late Cenozoic ca. 2 Ma-780 ka and 250-70 ka. The dams are strongly asymmetric, with steep, blunt escarpments facing up valley and long, low slopes down valley. None of the dams shows evidence of catastrophic failure; all blocked the river and diverted water over or around the dam crest. The net effect of the dams was therefore to inhibit rather than promote incision. Once incision resumed, most of the intracanyon fl ows were incised relatively rapidly and therefore did not exert a lasting impact on the river valley profi le over time scales >10 6 yr. The net longterm incision rate from the time of the oldest documented lava dam, the Bogus Rim lava dam (≤1.7 Ma), to present was 0.18 mm/yr, but incision rates through or around individual lava dams were up to an order of magnitude greater.At least three lava dams (Bogus Rim, Saddle Butte, and West Crater) show evidence that incision initiated only after the impounded lakes fi lled completely with sediment and there was gravel transport across the dams. The most recent lava dam, formed by the West Crater lava fl ow around 70 ka, persisted for at least 25 k.y. before incision began, and the dam was largely removed within another 35 k.y. The time scale over which the lava dams inhibit incision is therefore directly affected by both the volume of lava forming the dam and the time required for sediment to fi ll the blocked valley. Variations in this primary process of incision through the lava dams could be infl uenced by additional independent factors such as regional uplift, drainage integration, or climate that affect the relative base level, discharge, and sediment yield within the watershed.By redirecting the river, tributaries, and subsequent lava fl ows to different parts of the canyon, lava dams create a distinct valley morphology of fl at, broad basalt shelves capping steep cliffs of Tertiary sediment. This stratigraphy is conducive to landsliding and extends the effects of intracanyon lava fl ows on channel geomorphology beyond the lifetime of the dams.

[1] The high relief of the modern Rocky Mountain landscape formed in the late Cenozoic by downcutting of a fluvial network that links a series of easily eroded sedimentary basins across relatively resistant crystalline cores of adjacent ranges. Using a numerical model of fluvial erosion and the flexural isostatic response to the associated unloading, we first calculate the expected pattern and pace of incision caused by rock uplift related to migration of the Yellowstone hot spot and to growth of the northern portion of the Rio Grande rift. Calculated incision rates are <60 m/Myr, and total depth of erosion of sedimentary basins is <300 m, well below the long-term incision rates and amounts of erosion interpreted from the geologic record. Broad-scale tilting of the region toward the east, accomplished by a gradient in rock uplift of $1 km along the north-south axis of the central Rockies, declining to zero 1000 km to the east, can account for the additional erosion needed to match observations. In each modeling scenario, stream incision is nonsteady, with rock uplift outpacing erosion for <1 Myr in perimeter basins and 1-5 Myr in interior basins. Three factors dominate the spatial and temporal pattern of regional landscape evolution: (1) the time since uplift began, (2) the uplift pattern, and (3) the distribution of relatively resistant bedrock within the region. Our results suggest that the spatial variability in late Cenozoic exhumation can be explained by a long-lived transience in the stream network response to these various late Cenozoic geophysical events.

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[1] Low-temperature thermochronologic data are useful for estimating mountain erosion rates because late stage cooling in mountain ranges often reflects exhumation via geomorphic erosion processes. Traditional interpretations of thermochronologic data assume steady, uniform erosion, conditions often violated in real mountain ranges. Here I examine whether unsteady or nonuniform erosion histories are detectable in patterns of erosion rate estimates made by traditional means. To do so, I numerically model the subsurface temperature field in a two-dimensional section of Earth's crust under various illustrative geomorphic scenarios. I generate simulated cooling ages within two isotopic systems, apatite fission track (FT) and (U-Th)/He, (He) at two landscape locations, ridge tops and valley bottoms. From these ages I make ''field estimates'' of erosion rates using the commonly employed elevation dependence and mineral pair methods. Calculations show that if errors on erosion rate estimates are kept small by replicate or transect sampling, many unsteady or nonuniform erosional histories do produce distinctive patterns of erosion rate estimates. For example, temporally increasing erosion rates produce poor agreement between erosion rate estimates made with a single method, while decreasing erosion rate produces good agreement between estimates made with a single method. Nonuniform erosion that decreases relief can produce erroneously high erosion rate estimates based on the elevation dependence method but also accurate and relatively precise mineral pair-based estimates. A history of decreasing relief can be distinguished from one of steady, uniform erosion by (1) close agreement between mineral pair estimates and poor agreement between elevation dependence estimates and (2) the magnitude of the difference between FT and He ages. The insights gleaned from these analyses can guide forward modeling of topographic and thermal evolution.