StreaiTI Processes, Landscapes, Mass Wastage, and Flood Hazards CONTRIBUTING AUTHORS Pamela] .W Gore • Georgia Perimeter College Richard W Macomber • Long Island University-Brooklyn Cherukupalli E. Nehru • Brooklyn College (CUNY) OBJECTIVES AND ACTIVITIES INTRODUCTION A. Be able to use topographic maps and stereo­ It all starts with a single raindrop, then another and an­ grams to describe and interpret stream valley other. As water drenches the landscape, some soaks into shapes, channel configurations, drainage the ground and becomes groundwater. Some flows patterns, depositional features, and the evolu­ over the ground and into streams and ponds of swface tion of meanders. water. The streams will continue to flow for as long as ACTIVITY 1: Introduction to Stream they receive a water supply from additional rain, Processes and Landscapes melting snow, or base flow (groundwater that seeps ACTIVITY 2: Meander Evolution on the into a stream via porous rocks, fractures, and springs). Rio Grande Perennial streams flow continuously throughout the year and are represented on topographic maps as blue B. Understand erosional and mass wastage lines. Intermittent streams flow only at certain times of processes that occur at Niagara Falls, and be the year, such as rainy seasons or when snow melts in able to evaluate rates at which the falls is retreat­ the spring. They are represented on topographic maps ing upstream. as blue line segments separated by blue dots (three blue ACTIVITY 3: Mass Wastage at Niagara Falls dots between each line segment). All streams, perennial C. Be able to construct a flood magnitude/frequency and intermittent, have the potential to flood (overflow graph, map floods and flood hazard zones, and their banks). Floods damage more human property in assess the extent of flood hazards along the Flint the United States than any other natural hazard. River, Georgia. Streams are also the single most important ACTIVITY 4: Flood Hazard Mapping, Assess­ natural agent of land erosion (wearing away of the ment, and Risk land). They erode more sediment from the land than wind, glaciers, or ocean waves. The sediment is trans­ ported and eventually deposited, whereupon it is STUDENT MATERIALS called alluvium. Alluvium consists of gravel, sand, silt, and clay deposited in floodplains, point bars, Pencil, eraser, ruler, calculator, 12-inch length of channel bars, deltas, and alluvial fans (Figure 1). string. From Laboratory 11 of Laboratory Manual in Physical Geology, Ninth Edition, American Geological Institute, National Association of Geoscience Teachers, Richard M. Busch, Dennis Tasa. Copyright © 2011 by Pearson Education, lnc. Published by Pearson Prentice Hall. All rights reserved. 221 Stream Processes, Landscapes, Mass Wastage, and Flood Ha zards Heads of Main stream tributaries B. CHANNEL TYPES IN MAP VIEW Uplands A Tributaries X, Y ~ Boundary of Tributary X drainage A. STREAM basin (dashed line) DRAINAGE SYSTEM Yazoo tributary Marsh Point bar building to right 1 C. FLAT-BOTTOMED VALLEY WITH MEANDERING STREAM CHANNEL Channel bar D. FLAT-BOTTOMED VALLEY WITH BRAIDED CHANNELS AND SEDIMENT OVERLOAD FIGURE 1 General features of stream drainage basins, streams, and stream channels. Arrows indicate current flow in main stream channels. A. Features of a stream drainage basin. B. Stream channel types as observed in map view. C. Features of< meandering stream valley. D. Features of a typical braided stream. Braided streams develop in sediment-choked streams. 222 Stream Processes, Landscapes, Mass Wastage, and Flood Hazards Therefore, stream processes (or fluvial processes) This pattern develops on conical hills, such as are among the most important agents that shape volcanoes and some structural domes. Earth's surface and cause damage to humans and • Centripetal pattern-channels converge on a cen­ their property. tral point, often a lake or playa (dry lake bed), at the center of a closed basin (a basin from which surface water cannot drain because there is no outlet valley). PART A: STREAM PROCESSES AND LANDSCAPES • Annular pattern-a set of incomplete, concentric rings of streams connected by short radial chan­ Recall the last time you experienced a drenching rain­ nels. This pattern commonly develops on eroding storm. Where did all the water go? During drenching structural domes and folds that contain alternat­ rainstorms, some of the water seeps slowly into the ing folded layers of resistant and nonresistant ground. But most of the water flows over the ground rock types. before it can seep in. It flows over fields, streets, and sidewalks as sheets of water several millimeters or • Trellis pattern-resembles a vine or climbing rose centimeters deep. This is called sheet flow. bush growing on a trellis, where the main stream Sheet flow moves downslope in response to the is long and intersected at nearly right angles by pull of gravity, so the sheets of water flow from streets its tributaries. This pattern commonly develops and sidewalks to ditches and street gutters. From the where alternating layers of resistant and nonresis­ ditches and storm sewers, it flows downhill into small tant rocks have been tilted and eroded to form a streams. Small streams merge to form larger streams, series of parallel ridges and valleys. The main larger streams merge to form rivers, and rivers flow stream channel cuts through the ridges, and the into lakes and oceans. This entire drainage network, main tributaries flow perpendicular to the main from the smallest upland tributaries to larger streams, stream and along the valleys (parallel to and to the largest river (main stream or main river), is called between the ridges). a stream drainage system (Figure lA). • Deranged pattern-a random pattern of stream channels that seem to have no relationship to un­ Stream Drainage Patterns derlying rock types or geologic structures. Stream drainage systems form characteristic patterns Drainage Basins and Divides of drainage, depending on the relief and geology of the land. The common patterns below are illustrated The entire area of land that is drained by one stream, or in Figure 2: an entire stream drainage system, is called a drainage basin. The linear boundaries that separate one • Dendritic pattern-resembles the branching of a drainage basin from another are called divides. tree. Water flow is from the branch-like tributaries Some divides are easy to recognize on maps as to the trunk-like main stream or river. This pattern knife-edge ridge crests. However, in regions of lower is common where a stream cuts into flat lying lay­ relief or rolling hills, the divides separate one gentle ers of rock or sediment. It also develops where a slope from another and are more difficult to locate stream cuts into homogeneous rock (crystalline precisely (Figure lA, dashed line surrounding the igneous rock) or sediment (sand). Tributary X drainage basin). For this reason, divides cannot always be mapped as distinct lines. In the • Rectangular pattern-a network of channels absence of detailed elevation data, they must be rep­ with right-angle bends that form a pattern of resented by dashed lines that signify their most prob­ interconnected rectangles and squares. This able locations. pattern often develops over rocks that are You may have heard of something called a fractured or faulted in two main directions that continental divide, which is a narrow strip of land are perpendicular (at nearly right angles) and dividing surface waters that drain in opposite break the bedrock into rectangular or square directions across the continent (Figure 3). The blocks. The streams erode channels along the continental divide in North America is an imaginary perpendicular fractures and faults. line along the crest of the Rocky Mountains. Rainwa­ • Radial pattern-channel flow outward from a ter that falls east of the line drains eastward into the central area, resembling the spokes of a wheel. Atlantic Ocean, and rainwater that falls west of the Water drains from the inside of the pattern, where line drains westward into the Pacific Ocean. the "spokes" nearly meet, to the outside of the Therefore, North America's continental divide is pattern (where the "spokes" are farthest apart). sometimes called "The Great Divide." 223 Stream Processes, Landscapes, Mass Wastage, and Flood Hazards STREAM DRAINAGE PATTERNS Dendritic: Irregular pattern of channels that branch like a tree. Rectangular: Channels have right-angle bends developed along Develops on flat lying or homogeneous rock. perpendicular sets of rock fractures or joints. Radial: Channels radiate outward like spokes of a wheel from a Centripetal: Channels converge on the lowest point in a closed high point. basin from which water cannot drain. Annular: Long channels form a pattern of concentric circles Trellis: A pattern of channels resembling a vine growing on a connected by short radial channels. Develops on eroded domes trellis. Develops where tilted layers of resistant and nonresistant or folds with resistant and nonresistant rock types. rock form parallel ridges and valleys. The main stream channel cuts through the ridges, and the main tributaries flow along the valleys parallel to the ridges and at right angles to the main stream. Deranged: Channels flow randomly with no relation to underlying rock types or structures. FIGURE 2 Some stream drainage patterns and their relationship to bedrock geology. 224 FIGURE 11.3: Lake Scott, Kansas (1976) 0 .5 1 kilometer hE3E"3E3E31 'I• 'h 3067 41 r I I I --~ ~~~-~J~ ~ II II II I I' II ,; II I• __ .,1 ___ , _ , 1 II I II : / l ~ 1 II ] 0 66 •, ) \ \ _ _L ' \ _____ ! ~~30~247. ----~. J01J 1 I ( I / / I I I I I I j I )j "1 -- -- -'8.?61 1-7' --. I I 14/ I I !----·' I ' I \ - r ! I 1 I, I I I /I I I I I. I \ f. { n I i ! 'I I I lsM ("i I l j / '.}_041 I L' --~""=.---,-- --.--"""==-- ·-'~ ~ - J' 3025 1 rr.