Landscapes Shaped by Stream Erosion

Objectives Outline effects of geologic structure, lithology, tectonics and climate on stream erosion Introduce types of drainage networks and their origin Examine cases of stream erosion overcoming control of geologic structure Review major theories of long-term landscape development

Introduction Rivers erode landscapes—processes occur at same time Widening and deepening of valleys Slope modification Mass movements Exposure of buried Stream modified landscapes more extensive than other types

Factors Affecting Patterns of Stream Erosion--Climate Debate continues about role of climate in landscape evolution Yet landscapes imprinted by past climate. For example, glacial period Rivers with great sediment load drained ice sheets

Factors Affecting Patterns of Stream Erosion--Lithology Rocks have different hardness and resistance to weathering and erosion Quartzites very resistant Mount Monadnock Shales/mudstone not resistant Limestone resistant in dry climate but not in wet one

Factors Affecting Patterns of Stream Erosion--Geologic Structure Modified by streams to form landscape Hogbacks, Cuestas Ridges & Valleys—Appalachian Mountains

Factors Affecting Patterns of Stream Erosion--Geologic Structure Modified by streams to form landscape Domes—Black Hills Faults Produce scarps Uplifted blocks modified and valley s sediment –filled

Factors Affecting Patterns of Stream Erosion--Geologic Structure Modified by streams to form landscape Faults Produce scarps Uplifted blocks modified and valleys filled with sediment

Factors Affecting Patterns of Stream Erosion--Geologic Structure Extrusive Igneous structures Volcanic landforms Mesas and buttes Intrusive Igneous structures Domes of granite

Tectonic Activity Produces high mountains Accelerates erosion, available potential energy is greater Himalayas show steep stream slopes Appalachians also possess steep slopes

Drainage Patterns Influenced by structure and rock type High drainage density related to high erosion rates Badlands Radial Drainage Develop mostly on volcanic cones Annular Drainage Develops on domes Trellis Drainage Develops on parallel folded or dipping sedimentary rocks of differing hardness Rectangular Drainage Forms where joints or faults dominate Dendritic Drainage Forms on extensive batholiths or flat-lying sedimentary rocks

Overcoming Geologic Structure Rivers may cut across geologic structure Superimposed Streams Stream present above buried mountain range Erodes downward as overlying rock is removed Stream maintains course regardless of lithology and structure Antecedent Streams Stream cutting across uplifted ridge River keeps pace with uplift

Stream Capture Stream captures another stream Develops through headward erosion Example Upper Kafue River captured by Lower Kafue River

Long-Term Landscape Evolution Davis proposed cycle of erosion Elements include geologic structure, geographic process and time or stage Rapid tectonic uplift of area Uplifted areas shaped by erosion during long time periods Most hills eroded away to form peneplain Penck proposed uplift and erosion occur simultaneously not in sequence Hack proposed dynamic equilibrium Ruggedness of topography does not change