River Valleys

1. River Valleys Generally speaking, it is true that river valleys in upper reaches tend to be narrow and steep, while in lower reaches valleys are wider and have more gentle slopes. The steeper slopes in upper reaches promote vertical and headward erosion, rather than lateral erosion. Upper Lower Harry Williams, Geomorphology

2. The exact shape of the valley is also determined by mass wasting, which depends on hillslope processes and rock strength. Harry Williams, Geomorphology

3. In lower reaches, 2 important changes usually occur - the river becomes an alluvial channel (flows through its own alluvium); and the channel has a greater tendency to erode laterally, leading to a wider, flat bottomed valley. Upper reaches- bedrock channel,fairly straightstreams, no deposition, steepslopes. Harry Williams, Geomorphology

4. Middle reaches - streams begins to erode laterally, carving out a wide flat floodplain, filled with alluvium (stream sediment). More meandering, gentler slopes. deposition Alluvium Bedrock Harry Williams, Geomorphology

5. Lower reaches - floodplain wider, more meandering, gentler slopes. Harry Williams, Geomorphology

6. The width of the valley created is dependent on the resistance of the surrounding rock to lateral erosion - so different rock types undergo differential lateral erosion (e.g. Austin Chalk, Eagle Ford Shale, Taylor Marl in Dallas). hills Harry Williams, Geomorphology

7. Channel Long ProfilesRecap: streams are steep in their upper reaches, more gently sloping in their lower reaches... Harry Williams, Geomorphology

8. So, streams naturally tend to develop concave-upward long profiles - this comes about because of the need to balance the transporting ability of the river and the input of sediment i.e.: 1. in the upper reaches of a network, discharge is low and sediment is coarse - therefore steep slopes are required to give the river sufficient energy to transport the sediment (if the slope was too low, sediment supplied by mass wasting would build up the bed of the channel and increase the slope: if the slope was too steep, the excess energy would erode the bed, lowering the slope and causing headward erosion). 2. in the lower reaches of a network, discharge is high and sediment is finer (because of abrasion) - therefore gentle slopes are sufficient to give the river enough energy to transport the sediment. Harry Williams, Geomorphology

9. The bottom of the profile is aligned to the stream's BASE LEVEL (the level to which the stream erodes down to, but no lower). Harry Williams, Geomorphology

10. For many large rivers, the ocean represents base level, but for smaller streams - lakes, larger streams, resistant rock outcrops can all act as LOCAL BASE LEVELS. In reality, of course, the smooth concave profile is rarely achieved: irregularities such as resistant rock outcrops result in a "step" in the profile, marked by waterfalls or rapids. Harry Williams, Geomorphology

11. Streams will try to adjust their long profile until the smooth concave shape is achieved. The bed of a stream can be lowered by erosion or raised up by deposition. These processes are known as DEGRADATION and AGGRADATION respectively. Degradation to lower profile Harry Williams, Geomorphology

12. Aggradation to raise profile Harry Williams, Geomorphology

13. These processes also operate over the short term in response to precipitation inputs (i.e. a large storm causes sediment to be eroded from the channel bed and then deposited again when the storm ends and flow decreases). Movement of sediment through a river system therefore tends to be discontinuous - the sediment is stored within the river system between storms that are big enough to cause the sediment to be re-entrained and moved further downstream - in many systems, it can take hundreds or thousands of years for sediment to move all the way through. Over the long term however, if as much sediment as is supplied can be moved through, the river is said to be graded (i.e. there is a balance between input and output of sediment to the river system), and the shape of the channel over the long-term may remain essentially unchanged (a graded long profile). Harry Williams, Geomorphology

14. For this reason, many channels flow through their own deposits - these are ALLUVIAL CHANNELS. These occur particularly in lower reaches where deposition is more likely to occur; in the upper reaches channels may be cut directly into the underlying rock, forming BEDROCK CHANNELS. ALLUVIAL CHANNEL Harry Williams, Geomorphology

15. FLOODPLAINS Since flow is faster around the outside of a bend, meanders tend to shift sideways by eroding their outer bank (faster flow -> erosion) and at the same time depositing sediment on their inner bank (slower flow -> deposition). Harry Williams, Geomorphology Harry Williams, Geomorphology 15

16. The resulting plain is the FLOODPLAIN, because it is also the area that is flooded by the river when it overflows its banks. Harry Williams, Geomorphology Harry Williams, Geomorphology 16

17. Harry Williams, Geomorphology Harry Williams, Geomorphology 17

18. this also adds to the deposition of alluvium, because finer suspended sediment (silt and clay) is carried in over-bank flows and deposited on the floodplain. So floodplains are built from point bars and flood deposits. Floodplains are one of the major storages for sediment within the fluvial system. As noted last week, the depostion of sediment in the floodplain is a form of aggradation, but it is possible for the river to be in a state of dynamic equilibrium, if the amount of deposition is balanced by the amount of erosion, so there is no net gain or loss and sediment continues to be transported through the system. Harry Williams, Geomorphology Harry Williams, Geomorphology 18

19. River Terraces River terraces are benches on the valley side formed when a river cuts into a former floodplain. Terraces can be formed in bedrock, but most terraces result from AGGRADATION followed by INCISION of the river valley - i.e. the formation of a VALLEY FILL (deposition of sediment raising the valley floor), followed by downcutting (caused by environmental changes). Harry Williams, Geomorphology Harry Williams, Geomorphology 19

20. These terraces consist of alluvial deposits (sand, mud) and may contain relict floodplain features (e.g. filled-in oxbow lakes). Valley fills form when the input of sediment to the river system is too large to be transported - for example, a climate change that results in more weathering and erosion. Perdenales Terrace Modern point bar Harry Williams, Geomorphology Harry Williams, Geomorphology 20

21. In this area, large scale aggradation followed by incision and terrace formation occurred widely during the Pleistocene epoch (the last ice age, beginning 3 million, ending 10,000 years ago). Thus, many Quaternary (= Pleistocene + Holocene epochs) fluvial deposits are in the form of terraces. As climates fluctuated during the Quaternary, the balance between sediment supply and river discharges changed: generally speaking, periglacial (marginal glacial areas) climates produced large amounts of sediment (by frost action, for example) - often more than could be transported, causing aggradation. During interglacials, sediment supplies were reduced and rivers incised their valley fills, causing terraces. Harry Williams, Geomorphology Harry Williams, Geomorphology 21

22. Qt = Quaternary Terrace Harry Williams, Geomorphology Harry Williams, Geomorphology 22

23. In this area the Hickory Creek Terrace (recently named by Reid Ferring) is one of the best defined. Probably formed 76-30 thousand years ago, the Hickory Creek Terrace represents slow aggradation over a long period, resulting in a very wide floodplain with a gradient less than that of the present-day floodplain. Harry Williams, Geomorphology Harry Williams, Geomorphology 23

24. Gradients of Hickory Creek terrace and modern Elm Fork floodplain. Harry Williams, Geomorphology Harry Williams, Geomorphology 24

25. The Hickory Creek terrace is responsible for discontinuous, relatively flat-topped benches, standing above the regular floodplain of local streams e.g. in the vicinity of Green Valley adjacent to the Elm Fork. Another consequence of climate and river discharge changes is that many streams can be "underfit" or "misfit", meaning that they occupy valleys created by larger rivers in the past. Although the controls on valley width are still uncertain, generally speaking, the larger the river, the wider it's valley tends to be; and, most valleys are not much wider than the width of the river's meander belt. Many streams is this area are apparently underfit, because their valleys were formed by larger rivers in the past, which were also responsible for forming larger floodplains - the remnants of which form the present-day terraces. Harry Williams, Geomorphology Harry Williams, Geomorphology 25