IntroductionStream GradientDividesAlluvial FansStream Systems in Humid ClimatesEarly Stages of Stream DevelopmentMature StageRejuvenated SystemsEXERCISE 3: FLUVIAL (STREAM) SYSTEMSGY301 GeomorphologyFluvial Erosion LandformsIntroductionRunning water is by far the most important erosional agent on Earth. Even in aridclimates, it is the primary agent of erosion. This is evident from the abundance of streambeds found throughout desert areas. Although these areas receive little rain throughout theyear, the few days of rainfall produce flash floods that erode and transport tremendousamounts of material because of the lack of vegetation.Stream GradientThe stream gradient refers to the slope of the land surface upon which a streamflows. The stream gradient is calculated by comparing two points of known elevation alongthe stream's course with the horizontal map distance between the two points:The gradient is generally given in units of feet per mile (or m/km). When calculating agradient do not try to convert the numerator and denominator of the fraction to equivalentunits unless explicitly requested to do so. For example, if two points had an elevationdifference of 200 feet according to the contour data, and the two points were 4 miles apartas measured along the stream course,the resulting gradient would be 50 feet per mile. Thestream gradient bears a direct relationship with topographic contour spacing; if the streamcourse runs across many contours in a short distance the gradient will be high, whereasstreams flowing across flat plains will have low gradients. The calculation of a streamgradient may be made from any two points along the stream since elevations may beinterpolated from contours and map distance is determined by the map scale, however, itis more convenient to pick points that lay on contour lines to determine elevations.The erosional power of a stream is proportional to the discharge of the stream whichis, in turn, primarily a function of the size of the stream and the velocity of the stream water.Because an increased stream gradient is usually correlated with a higher velocity, thehigher gradient streams will erode their channels at a faster rate than other lower-gradientstream systems. Page -1-GY301 GeomorphologyFluvial Erosion LandformsDividesA stream divide separates stream drainage basins. The separation between twodrainage basins is a linear topographic high such as a a ridge or plateau; on opposite sidesof the ridge streams of the different drainage systems flow in opposite directions. Todelineate the divide one could draw a pencil line on a topographic map that connectspoints of maximum elevation along the ridge crest. This line will divide the two drainagesystems because all streams will drain away from this line.A stream system is typically made up of a dendritic pattern of tributaries that draina large drainage basin. A ridge will separate individual drainage basins along a divide. Adivide may be recognized on a topographic map as a ridge across which streams tend toflow in opposite directions. The continental divide west of the front range of the RockyMountains is but one example of a large divide. The divides are important to earthscientists because the water resources of one drainage basin are normally isolated fromthose of another drainage basin.Alluvial FansAlluvial fans are common in arid to semiarid climates and are depositional productsof running water that is, surprisingly, the primary erosional agent in these types of climates.Since arid landforms do not have the extensive vegetative cover found in humid climatesthe soil horizon is not well anchored in these types of terrains. Therefore, during rare butsubstantial rain or snow-melt the soil horizon is eroded very quickly. The drainage systemof a desert landform is not well-integrated so most of the transported material is not moveda large distance. The deposited sediment tends to collect at slope breaks where thecapacity of the stream is exceeded. These conical aprons of sediment that build-up alongthe flanks of mountain ranges are termed alluvial fans. Alluvial fans are easily recognizedbecause of the characteristic shape of contour lines that cross the fan. The contours tendto form evenly spaced semicircular geometries because of the conical shape of the alluvialfan surface. Figure 1 is a portion of a USGS 7.5' quadrangle that contains an alluvial fanlandform feature. Note that the contours are evenly spaced about the alluvial fan, indicatinga relatively constant slope angle. Also note that a topographic profile taken perpendicularto evenly spaced contours produces a planar sloping surface. Natural landforms in aridregions will not possess contours that are perfectly spaced, therefore, the surface is usuallyslightly concave up in form.Because the sediment incorporated into an alluvial fan has traveled a relatively shortdistance, the particles that compose the sediment tend to be angular and poorly sorted ascompared to sediments deposited by streams in a humid climate. Because the particlesPage -2-GY301 GeomorphologyFluvial Erosion Landformshave a short transport residence time, the materials composing the sediment may be onlyslightly affected by chemical weathering. This is also due to the low humidity in the aridclimate. Streams that flow on top of alluvial fans are typically intermittent and form braidedchannels since they are often overloaded with sediment.As erosion wears down mountain ranges in arid climates, alluvial fans build at thebase of the mountain ranges. The alluvial fans become larger over time until several fanswill merge to form a broad apron of debris that is termed a bajada. The bajada isrecognized on a topographic map by the straight rather than crescent-shaped contour linesthat have a constant spacing. In Figure 2 the bajada can be recognized from the relativelystraight and equally spaced contour lines. A mountain range in the mature stage of the aridcycle of erosion may be entirely covered by a bajada on each flank. The elevated remnantsof a mountain range surrounded by an expanding bajada is termed inselberg, which is aGerman derivation meaning literally "island mountain". Several inselbergs are present astopographic hills in Figure 2.The low-lying valleys that lie between mountain ranges are the sites where runofffrom alluvial fans collect. The collecting water forms broad shallow lakes during the