Stream Drainage. Patterns of Stream Development

Transcription

1 Stream Drainage Patterns of Stream Development Unless otherwise noted the artwork and photographs in this slide show are original and by Burt Carter. Permission is granted to use them for non-commercial, non-profit educational purposes provided that credit is given for their origin. Permission is not granted for any commercial or for-profit use, including use at for-profit educational facilities. Other copyrighted material is used under the fair use clause of the copyright law of the United States.

2 1 Drainage Basins and Divides

3 Runoff through a major stream originates in smaller and smaller tributaries, and ultimately from sheetwash and springflow off the land bordering all those streams. The entire region whose runoff is funneled through a particular stream and into the next bigger stream or the ocean is called the drainage basin of that stream. There always exists some height of land that separates runoff into ione stream from runoff into another. For example, there is a somewhat curvy line we could draw across campus from roughly the softball fields to roughly the climbing tower near the golf course, separating the front of campus from the back. Rain that falls on the back part of campus drains through Willett Creek, that on front campus drains directly into Mill Creek. This height of land is called the drainage divide between two drainage basins. The next slide shows the major drainage basins of Georgia and the trunk streams through which they ultimately reach the ocean (along with the more immediate large river they drain through). The approximate drainage divides are shown by the dashed lines.

4 Mississippi (Tennessee) Tensaw (Alabama) Savannah Apalachicola (Chattahoochee/ Flint) Oconee Altamaha Satilla Ochlockonee St Marys Suwanee Base map from Delorme mapping software.

5 Of course there are a hierarchy of drainage basins that contribute to a single trunk stream. The red solid line on the map shows the drainage divides of the Chattahoochee and Flint basins that feed the Apalachicola. In turn, the black dashed lines outline some of the sub- basins of the Flint, including (S to N) the Spring Cr., Ichawaynochaway Cr., Kinchafoonee Cr., and Muckalee Cr. basins. Every small tributary you see has its own small drainage basin. Base map from Delorme mapping software.

6 We can characterize a drainage basin by the complexity of the hierarchy of tributaries. The smallest permanent ones are called first order tributaries. The junction of two first order tributaries makes a second order, and so on. the order of the trunk that enters the ocean is the order of the entire drainage basin. For example, if the largest stream in this map view entered the ocean the basin would be a third order basin. A small, young stream like the St Marys (S on the map) will have a low order. A large, old drainage system like the Altamaha (A on the map) will have a much higher order. In fact, we can make a good guess about the relative ages of basins based on their orders. A S Base map from Delorme mapping software.

7 Look carefully at the heads of these actively forming gullies in the Great Plains (in the oval, for example) and you ll see the branching pattern in their intermittent channels (on a smaller scale) that you also see in the permanent streams below them (blue dashed lines). These streams (and all others) are actively obeying Playfair s Law and creating their own drainage. Not all of the (now) intermittent channels will be equally successful, but as they grow longer by headward erosion some will eventually reach the same lower level that their downstream trunks cross and become permanent. At that point they will be longer than they are now, will receive runoff from a longer valley (and its sides), and will have evolved into permanent streams of the first order. What are now first order streams (the smaller branches shown in blue) will then be second order streams and the entire drainage basin (the White River, locally; the Mississippi, ultimately) will be one order higher than it is now! Badlands National Park, SD. Image from Google Maps

8 2 Drainage Patterns

9 As streams enlarge their valleys by headward erosion they create several different patterns of drainage depending upon what kind of landscape they cross and what kinds of rocks underlie that landscape. If the land slopes fairly generally in the same direction, and if the rocks beneath it are all about equally hard (or equally soft, depending on your point of view) the exact branching pattern will be a result of random headward erosion. On the other hand, if there is some odd slope direction, or if the rocks below are not equally hard then the headward erosion may be channeled into preferred directions and the result will be non-random headward erosion.

10 All the drainage we ve focused on so far is across gently sloping ground, tilted generally in more or less the same direction, and underlain by virtually flat sedimentary rocks or consistently hard igneous/metamorphic rocks. This allows the streams to erode headward in random fashion and to create a drainage pattern called dendritic drainage. Dendritic comes from a Greek root (dendros) that means tree. Make sure you see how the branching pattern resembles that of a tree. A pecan tree is a good analogy. Base map from Delorme mapping software. Badlands National Park, SD. Image from Google Maps.

11 Drainage off of isolated distinct peaks, such as volcanoes, has its direction dictated by the peak itself. Such drainage is called radial. Volcan Popocatepetl, border of states of Peubla and Mexico, Mexico. Image from Google Maps.

12 In other cases, such as this sinkhole pond in northeastern Florida, the ground slopes inward toward the center of a closed depression. The resulting drainage pattern is called centripetal drainage. (Some lakes that receive drainage like this do have an outlet stream, in which case the drainage is not truly centripetal. Drainage in this example is via groundwater. No surface stream flows out of this lake. Lowry Lake, southwestern clay Co., FL. Base image from Google Maps.

13 Regions like the western Appalachians, in which the rocks are folded, have long narrow ridges separated by long narrow valleys. The landscape is said to have valley and ridge topography. In fact, this area of the Appalachians is the Valley and Ridge Province. The left-hand map shows the wooded ridges and developed valleys near Sweetwater, TN. In this case long roughly parallel trunks in the valleys are fed by short, often intermittent tributaries from the adjacent ridges. Rarely do streams cross a ridge, though it can happen in a couple of ways, as we will see. The right-hand map is the same region (with the contrast enhanced) to show the drainage. Trunk streams run NNE/SSW and tributaries come in at almost right angles to them. The tributaries are larger and have more lower level tributaries in wider valleys. This type of drainage is called trellis drainage.

14 In areas where rocks are cracked, or fractured, or jointed streams find headward erosion easier along the joints rather than across the grain, and so the drainage tends to follow them. Trunks and tributaries tend to be straight for some distance and then make bends at specific angles. Tributaries meet trunks at those same specific angles. This area in eastern Utah shows streams meeting at about 90 and about 60. A few such junctions are indicated, but you should be able to spot plenty of others. This type of drainage is called angular drainage (or rectangular drainage if the angles are consistently at ~90).

15 3 Stream Capture or Piracy

16 Headward erosion is typical of all streams except those that originate close to their base level in large swamps (Suwannee, Alapaha, and St Marys Rivers, in Georgia for example). Notice that the lower level tributaries of the Savannah, Altamaha, and Flint drainage basins are all close to, and eroding toward, the valley of the Chattahoochee. You can easily spot other examples of the smaller Coastal Plain streams in similar relationship to the Flint and Altamaha basins. You can probably project what will happen if the headward erosion continues. Base map from Delorme mapping software. In almost every case the smaller stream that threatens to capture another has a shorter distance to the sea. This means its gradient is higher. Water always follows the highest gradient to the sea, and so stream capture generally diverts the flow of the captured stream into the capturing stream s drainage basin, shrinking the one and expanding the other. stream capture is also called stream piracy.

17 The Alapaha River, which flows east of Valdosta, GA, has a much more extensive floodplain than a river of its size should have. In fact, its floodplain is a better fit to the Ocmulgee River, which makes a peculiar right-angle bend to the north of the Alapaha. The tiny Willacoochee River is tributary to the Alapaha as shown by the dotted blue line. Notice its orientation. One reasonable explanation for this interesting observation, compatible with all the components of the observation (geography, floodplain size, etc.) is stream capture shown on the next slide. Image from USGS Terraserver.

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19 3 Antecedent and Superposed Streams

20 In lab and lecture we ve seen some examples of streams (like the Madison River at Ennis, MT or the Flint River at Pine Mountain) that cross high ridges. Stream capture is one mechanism for a stream to cross one ridge, but many rivers actually cross entire mountain ranges, with multiple ridges and massifs. These must occur in other ways than simple capture. Image from Delorme Mapping Software

21 Image from Delorme Mapping Software Except for the Savannah and the Chattahoochee, the major rivers in the southern Appalachians all originate in the Piedmont, east of the mountains, and cross the entire range to get into the Tennessee or Ohio drainage systems.

22 In the central Appalachians the pattern is similar but reversed. Drainage from the James River to the Hudson originates west of (or within the western part of the mountains) and then crosses them, ridge after ridge in essentially straight-line paths. Jefferson wrote that he would like to have seen the Potomoc racing out of the mountians and crashing through the Blue Ridge, but that, in fact, never happened. Image from Delorme Mapping Software

23 What Jefferson was too early to realize was the long history of the river and the land it drains. These diagrams better fit the southern streams that drain into the Tennessee, but something similar also occurred in the north. Phase 1 of the story is when the Appalachians were built during a collision with another continent in the Carboniferous Period. The maximum deformation and uplift was to the west, creating a highland there. Streams began draining off both sides of those huge mountains, working to establish graded profiles. Phase 2 was late in mountain building and after the uplift ended completely. The mountains began to be unroofed or denuded by erosion and the sediment deposited in basins flanking the mountains. Some of this was deformed as it was deposited, but much of it is undeformed. Its deposition covered some of the structures and topography in and around the now shrinking mountains.

24 Now (phase 3) the streams have downcut and established grade, which in the former mountains and their flanks means that they have eroded through much of the bedrock and through the sediment they originally deposited to make their initial equilibrium profiles necessitated by their originally higher heads. This lowering of the streams onto a structurally and lithologically complex surface has allowed them to slice through ridges and massifs that they would have had to skirt, had they encountered those high elevations at the surface. Ironically, the New River of Virginia and West Virginia is a very old river, having flown in pretty much its present course since it began flowing off the young, Himalaya-scale Appalachians soon after they formed in the late Paleozoic. This process is called superposition of the stream.

25 The Yakima River crosses a series of ridges just north of Yakima, WA. It was not let down on these ridges from a higher surface. (That is, it is not superposed.) Instead the river is older than the ridges and managed to maintain its (meandering) course as the young ridges rose around it! Yakima River, Yakima Co., WA. Image from Google Maps

26 The Ventura River near Los Angeles has done the same thing. The various ridges have formed around it while it eroded through them. The fold ridge indicated has been well studied. It began forming a little over 200,000 years ago with an uplift rate of around 2cm/year. Though the rate slowed to ~1 cm per year by 80,000 years ago, and to ~½ cm/year presently the river maintained its original course, making its valley through the ridge as the ridge rose beside it! These streams are called antecedent streams because they predate the landscape they cross. Ventura River, Ventura Co., CA. Image from Google Maps

27 The San Juan River in southeastern Utah meanders through a series of canyons through ridges and mesas. It is not a superposed stream, but rather an antecedent one, like the Yakima and the Ventura. This explains the curious mix of old-age morphology (meandering) and youthful morphology (canyons). Meandering streams should occur close to base level and should have broad floodplains, neither of which is the case here. This part of the Colorado Plateau is being actively uplifted, and as it has risen the stream, which used to drain across a low flat surface, has eroded fast enough to maintain its former meandering form. Uplift that resets the river cycle and forces streams to renew their downcutting like this, is said to rejuvenate the stream to return it to a youthful morphology. Even though the stream is in a canyon, it can clearly still cut off meanders as seen on the picture below. Cutoff View of the picture above Goosenecks of the San Juan, San Juan Co., UT. Image from Google Maps