Investigation 2 – Concept Day
Watersheds: Investigation 2
In this Investigation, we continue discussing watersheds. The key concepts involve water turbidity and sedimentation of suspended materials in moving water as a function of water velocity and discharge rates. The presence of suspended particles in moving water is due to erosion of such material from surfaces over which the water has flowed. In addition, runoff from surrounding land introduces material into the water as well.
We will also focus on the effect of sedimentation on the mouths of several major rivers where they open into the ocean and discuss how this may form river deltas.
- As water moves over land surfaces, it is pulled along by gravity. Moving water is thus a form of kinetic energy. Depending on volume and velocity, enormous amounts of kinetic energy may be associated with moving water. The energy of moving water has the potential to significantly alter the land surfaces over which it flows in the form of erosion.
- As shown in this slide, erosion may have relatively subtle effects on the landscape (left) or major effects (right). The erosion and formation of rills on the field on the left may occur after one or two hard showers of rain. On the other hand, the erosion activity seen in the photograph of the Colorado River cutting into and carving out the Grand Canyon has involved many millions of years of erosion.
- Moving water contains particulate material of various sizes. This graph qualitatively shows that the size of particles that can be moved along with moving water is dependent on the discharge rate at which the water is moving.
- Not surprisingly, moving water at relatively low discharge rates rather easily carries very small particles, such as dissolved clay and silt. As particle size increases, more kinetic energy is required to carry them downstream. At very high discharge raters, even boulders can be driven downstream by the kinetic energy of rushing water.
- The Great Continental Divide runs through North, Central, and South America. The rain that falls to the west of the Divide will end up in the Pacific Ocean. The rain that falls to the east of the Divide will end up entering the Atlantic Ocean. To get to the Atlantic, the water may need to pass first through the Arctic Ocean to the north or Hudson Bay and the Labrador Sea to get there… but it will get there.
- This slide simply shows the major impact turbidity can have on a body of water. The sudden release of particles into water, which causes an increase in turbidity, may come from a variety of sources. Heavy rains upstream may wash dirt from the watershed into running water. This may be augmented by construction projects near the water’s banks.
- The effects of increased water turbidity are many. Obviously, water turbidity will decrease the depth to which sunlight penetrates the surface. This, in turn, would be expected to reduce the amount of photosynthesis that occurs by water plants and algae. Water turbidity can also impact animals who use gills and other water filtering structures to obtain dissolved oxygen from the water.
- In addition, the particles carried by moving water will eventually sink to the bottom, causing sedimentation. The effect of large sediment deposits on the biological community at the river’s bottom can be substantial.
- This slide focuses on the relationship between moving water velocity or discharge rate and turbidity and sedimentation rate. At high velocity and discharge rates, there is more kinetic energy available to carry material downstream in a suspended form. This increases turbidity. On the other hand, rapidly moving water discourages particles from settling at the bottom of the stream or river. Whether or not sedimentation occurs is largely a race between the kinetic energy of the moving water and the pull of gravitational energy on the particles. As we saw in previous slides, the size of the suspended particles will also determine if sedimentation occurs at any given discharge rate.
- At low water velocity and discharge rates, turbidity may be expected to be lower as suspended material in the water sinks to the bottom of the riverbed and adds to the sediment.
- This slide gives a schematic overview of the experiment you will perform in the Investigation 2 lab. You will mix a known mass of baking flour in water to make a turbid solution. Then, by setting a hotplate-stirrer to increasing RPMs, the velocity of the water in a beaker will be controlled. The effect of velocity on turbidity will then be determined by measuring light transmittance in the spectrophotometer.
- In the field, researchers can quickly and simply get a measure of water turbidity using a Secchi disk. The Secchi disk is typically 8 inches in diameter and painted black and white in the pattern shown. The disk is affixed to a line and sinks as it is lowered into the water.
- As explained on this slide, the researcher lowers the Secchi disk (from a dock, out of a boat, etc.) into the water until it just disappears from sight from the surface. This depth is marked on the line and the disk is lowered until it is completely out of sight. It is then raised until it is just seen again from the surface and this depth is marked on the line. The midpoint between the two marks on the line gives the “average Secchi depth”.
- While the Secchi disk method is not as accurate as taking water samples for spectrophotometric analysis (as you will perform in Investigation 2 lab), it is nonetheless a very easy and quick way to determine relative water turbidity.
- The green coloration of the water shown in this slide is due to the rather dense growth of suspended phytoplankton (microscopic plant-like organisms) such as algae. In some waters, phytoplankton is a major source of turbidity.
- We have been discussing water turbidity and sedimentation. There is perhaps no greater impact of turbidity and sedimentation than the formation of deltas at the mouths of certain rivers as they enter oceans, seas, or even some lakes.
- As we have suggested, when particle-laden turbid water slows down, suspend materials tend to sink to the bottom by gravity, forming sediment deposits. The flow of even very large rivers can be slowed as they reach the ocean or sea where water movement is much reduced compared to the river upstream (upstream is the direction from which flowing water is coming). Thus, much of the suspended particles that cause turbidity of the river water come out of solution and sediments near the mouth of the river. With time, the river tends to shallow and spread out at its mouth, causing reduced velocity and additional sedimentation.
- Over thousands of years, such river deltas can grow to very large sizes indeed. The Mississippi River delta, for example, covers approximately 6,259 square miles (16,200 square km). The largest river delta in the world is the Ganges River delta, with an area of approximately 41,000 square miles (105,000 square km). That is a greater area than the entire country of Iceland, Hungary, or Austria!
- This slide shows satellite views of four major river deltas.
- This slide provides a map of the world with the location of five major rivers indicated. As an exercise in simple geography, you may attempt to name the country in which the delta of these rivers is located and the body of water into which they empty:
- This slide shows a satellite image of the Mississippi River delta. Notice how the Mississippi River “fans out” as it arrives at the delta. Also, note the incredible amount of sediment that is carried from the delta into the Gulf of Mexico.
- Sediment that is carried by rivers is derived from the landforms in the watershed they flow through. As a result, river sediment is typically very high in nutrients. As a result, the lands in and around river deltas are some of the most fertile anywhere. The development and flourishing of ancient Egyptian civilization at its precise location was undoubtedly influenced by the rich Nile River delta.
- This final slide simply introduces the concept of watershed ecology and ecosystems. While we have spent very little time discussing the biologic inhabitants of watershed communities, they are nevertheless extremely important.
- Watersheds, and particularly their wetlands, are very rich in nutrients provided by a constant influx of river sediments carried from the reaches of the watershed. Plant and animal life of all kinds flourish in these important ecological systems. We will discuss elements of the estuary watershed ecosystems further in Investigation 3.