Slopes: solute processes and landforms

This chapter reviews research on solutes by fluvial geomorphologists in the period 1965 to 2000; growing links with biogeochemical research are emphasised later in the chapter. Brief reference is necessarily made to some research from before and after the study period. In relation to solutes, early research sought to relate short-term process observations to long-term landform evolution. However, very quickly, research moved into much more applied fields, less concerned with landforms and more with biogeochemical processes. The drainage basin became the focus of research with a wide range of interest including nutrient loss from agricultural and forested landscapes to dissolved organic carbon export from peatlands. In particular, the terrestrial-aquatic ecotone became a focus for research, emphasising the distinctive processes operating in the riparian zone and their contribution to river water protection from land-derived pollutants. By the end of the period, the scale and range of fluvial geomorphology had been greatly transformed from what it had been in 1965, providing a distinctive contribution to the broader field of biogeochemistry as well as an ongoing contribution to the study of Earth surface processes and landforms.

Long-term landform evolution

This chapter reviews major advances in studies of long-term landform evolution in the last few decades of the 20th century. These include in particular the development of etchplanation concept that evolved into a dynamic approach linking inheritance, environmental change, contemporary processes, realization of the importance of weathering mantles in deciphering landscape evolution at long timescales, recognition of vary ancient landforms, especially in the Southern Hemisphere, and insights into preglacial landscapes at high latitudes that survived successive glaciations.

Modeling glacial and fluvial landform evolution at large scales using a stream-power approach

Modeling glacial landform evolution is more challenging than modeling fluvial landform evolution. While several numerical models of large-scale fluvial erosion are available, there are only a few models of glacial erosion, and their application over long time spans requires a high numerical effort. In this paper, a simple formulation of glacial erosion which is similar to the fluvial stream-power model is presented. The model reproduces the occurrence of overdeepenings, hanging valleys, and steps at confluences at least qualitatively. Beyond this, it allows for a seamless coupling to fluvial erosion and sediment transport. The recently published direct numerical scheme for fluvial erosion and sediment transport can be applied to the entire domain, where the numerical effort is only moderately higher than for a purely fluvial system. Simulations over several million years on lattices of several million nodes can be performed on standard PCs. An open-source implementation is freely available as a part of the landform evolution model OpenLEM.

Geomorphic effects of recurrent outburst superfloods in the Yigong River on the southeastern margin of Tibet

Landslide dam outburst floods have a significant impact on landform evolution in high mountainous areas. Historic landslide dams on the Yigong River, southeastern Tibet, generated two outburst superfloods > 105 m3/s in 1902 and 2000 AD. One of the slackwater deposits, which was newly found immediately downstream of the historic dams, has been dated to 7 ka BP. The one-dimensional backwater stepwise method gives an estimate of 225,000 m3/s for the peak flow related to the paleo-stage indicator of 7 ka BP. The recurrence of at least three large landslide dam impoundments and super-outburst floods at the exit of Yigong Lake during the Holocene greatly changed the morphology of the Yigong River. More than 0.26 billion m3 of sediment has been aggraded in the dammed lake while the landslide sediment doubles the channel slope behind the dam. Repeated landslide damming may be a persistent source of outburst floods and impede the upstream migration of river knickpoints in the southeastern margin of Tibet.

A simple and efficient model for orographic precipitation

The influence of climate on landform evolution has received great interest over the past decades. While many studies aim at determining erosion rates or parameters of erosion models, feedbacks between tectonics, climate and landform evolution have been discussed, but addressed quantitatively only in a few modeling studies. One of the problems in this field is that coupling a large-scale landform evolution model with a general circulation model would dramatically increase the theoretical and numerical complexity. Only a few simple models are available so far that allow a numerical efficient coupling between topography-controlled precipitation and erosion. This paper fills this gap by introducing a quite simple approach involving two vertically integrated moisture components (vapor and cloud water). The interaction between both components is linear and depends on altitude. This model structure is in principle the simplest approach that is able to predict both orographic precipitation at small scales and a large-scale decrease in precipitation over continental areas without introducing additional assumptions. Even in combination with transversal dispersion and height-dependent evapotranspiration, the model is of linear time complexity and increases the computing effort of efficient large-scale landform evolution models only moderately. Even simple numerical experiments applying such a coupled landform evolution model show the strong impact of spatial precipitation gradients on mountain range geometry including steepness and peak elevation, position of the principal drainage divide, and drainage network properties.