Why does the width of a river change

Presenting the Investigation Question After the scene is set, introduce your students to the investigation question: How does a river change the land?

Have your students brainstorm ideas about how this investigation question could be investigated. How would you design an experiment that could be used to test the investigation question? What materials would you need? What would you have to do? What would you measure? How long would the experiment take? Assessing What Your Students Already Know Most students will have observed the movement of water in a stream or river, but they may not be aware of the effect of moving stream or river water on the land.

Here are some initial questions that your students can discuss, in pairs, in groups, and as a whole class: During big rainstorms, water often flows across the surface of the land. What does this water do to the soil and rock that it flows over? Where does water that flows over the land go?

What causes some rivers or streams to be brownish in color? What does the water flowing in a river or stream do to the rock or soil that surrounds the river? Have your students share their ideas with the class and record them as a list on the flipchart. They will create their models in stream tables containing sand. They will pour water into a coffee can. The water will flow through a rubber hose and into the streams.

They will observe how the flowing water affects the sand and shapes of the streams. Arrange students in groups. Provide the materials to each group.

Instruct students to cover their desktops with paper. Assign each group an image from the Stream and River Cards. Instruct students to draw a picture of their river systems on their observation sheets. Have them record their measurements on their observation sheets. Have students raise one end of their stream tables to a height of 10 cm. Instruct them to raise the end where the water reservoir will be pouring water into the rivers.

Students should use thin wooden blocks or books to raise the stream tables. Have students record their predictions on their observation sheets. Have students fill their water reservoirs and allow the water to flow through their streams. Have students record on their Observation Sheets the effect of the flowing water on the sand and shape of the streams. Instruct students to measure the widths of the stream beds at both ends and in the middle. This means that less of the water is in contact with the bed of the river and the mouth so there is less energy used to overcome friction.

Hence rivers flow progressively faster on their journey downstream. Width and depth increases as more water is added from tributaries. Gradient the slope of the land decreases as rivers flow because the river meanders across the land rather than erode into it and follow a straight path as it does in the source.

This means it covers a decrease in height over a longer distance the further downstream you get. Stone size decreases downstream and the stones get rounder and smoother as rivers erode the rocks progressively as the stones are transported downstream.

The processes of river erosion operate here. Hydraulic action - where the sheer force of the water erodes the stones, bed and banks of the river. Corrasion - where stone sin transport are thrown into the bed and the banks eroding them. The global dataset provides several pieces of compelling, but indirect, evidence for the threshold-limited channel model.

A direct test requires in situ measurement of the fluid entrainment threshold of bank-toe materials in a river with a sand bed and cohesive banks. We have developed a new instrument, the Mudbuster, that is specifically designed to overcome these shortcomings The principle, design, calibration, and testing of the Mudbuster are reported elsewhere S5 in which vegetation rooting depths were shallow compared with channel depth, to isolate sediment cohesion effects that could be measured directly with the Mudbuster.

These results confirm that, when the local entrainment threshold of cohesive banks is properly characterized e. We view this as a direct confirmation of the threshold-limited channel model. A Location of the Mullica river watershed in Wharton State Forest in the New Jersey coastal plain, with inset showing larger regional context image source: Google Earth. B Portion of the surveyed reach of the Mullica River. Red lines mark surveyed cross sections. Muddy bank and bed materials are shown in the bottom left and top right, respectively.

Photo credit: Kieran Dunne, Rice University. Thin blue lines show 28 independent measurements of bank-toe material, and solid line represents the median from those measurements; red line shows the mean value Pa determined from these data see Materials and Methods. A scale analysis based on hydrodynamic considerations of bar formation 46 successfully predicts the transition from single-threaded to multiple-threaded braided planform morphologies as a function of Q bf , S , W bf , and H bf Using our Eqs.

Our global dataset is composed almost exclusively of U. Last, we suggest that the downstream transition from a gravel- to a sand-bedded river may lead to a shift from bed to bank control, which may explain downstream changes in planform morphology such as that seen on the Fraser River fig.

S2 Inset images show typical braided morphology of the Waimakariri River, and single-threaded meandering Rio Purus image sources: Google Earth. Virtually, all rivers in this dataset should plot as single threaded. Selenga River delta channels were selected so as to be outside of the range of the backwater effect 48 , and additional rivers were reported by Schumm Our findings indicate that average alluvial river geometry may be predicted with knowledge of four parameters: bankfull discharge, slope, friction factor, and entrainment stress of the most resistant material.

Although friction factor varies among rivers, this variation is not systematic with any other parameter fig. S1 Results demonstrate that the Parker closure for gravel rivers can be extended to finer-grained systems by considering the most resistant bank material.

This model can describe the average geometry of dynamic alluvial rivers in nature, including those that transport bed sediment at a stress state far above threshold. We assert that this mean field approximation is not at odds with dynamic equilibrium theories; rather, they are two sides of the same coin. For some engineering applications, use of such a large space- and time-averaged model may not be useful.

In many contexts, the additional information required to develop and apply models more sophisticated than the threshold-limiting channel model is simply not available.

This framework also illustrates how changing riverbank composition, as well as interactions with biological forcings e. The simple models Eqs. The apparent success of the threshold-limited channel model, however, raises other intriguing questions. Sediment supply has been proposed to influence channel geometry 18 and also planform morphology We posit that there is an influence of sediment supply through its modulation of slope. On engineering decadal time scales, slope is often considered an independent variable because its time scale of adjustment is much slower than width and depth 16 , We suggest that changing sediment supply rate Q s has a slow influence on channel geometry through regrading of river slope.

Images of the channel were not necessarily taken at bankfull conditions, so bankfull extent was estimated on the basis of color variations with an approximate pixel resolution of 0. The global dataset we used has been presented elsewhere It contains measured channel geometry and discharge values associated with bankfull flow, i. Friction factor C f for each river was computed using a Darcy-Weisbach flow resistance relation Supplementary Materials. Channel geometry and bank composition data for rivers presented in Fig.

Data to produce the hydrograph magnitude-frequency curves Fig. Gravel-bedded rivers were the same rivers analyzed by Phillips et al. Because of heavy overlap between the gravel- and fine-grained rivers, only the mean for fine-grained rivers and SD for gravel-bedded rivers are shown. The Mullica River was selected for field work because of its proximity and the desired bed and bank properties for the study. Channel slope for the studied reach was determined over a 6-km stretch of river using a Trimble ProXH differential GPS sampling at 1 Hz from a boat fig.

We surveyed bankfull channel width and depth at 18 cross sections using a laser range finder fig. Calculations of bankfull stress at each location used the bankfull depth at each cross section and the reach-averaged slope.

At each cross section, the edge of the bank was identified in the field. Bank-toe erodibility measurements were made using the Mudbuster in situ erodibility tester following the procedures and calibrations outlined in another paper Fluid shear stress is systematically increased with the Mudbuster, while turbidity is measured using two photodiodes.

Increased turbidity measures as a voltage drop, which is expected to occur abruptly at a threshold fluid stress. While each measurement showed a voltage decline with increasing applied shear stress, determining a precise threshold was challenging due to noise. Variations of the voltage drop from measurements within a single cross section and measurements among different cross sections were of comparable magnitude. We thank J.

Pizzuto, J. Nittrouer, and C. Phillips for initial feedback that helped frame aspects of this paper, and D. Between sites 2 and 3, there was not as quite a dramatic change. Between sites 3 and 4 was a gradual yet steep rise in width. In the vicinity of sites, 2,3 and 4 there were man made bridges. The bridge at site 2 was used for crossing the road over the river. The bridge at site 3 was used for tractors and other machinery to enter a nearby field.

At site 4 there was an arched bridge, once again for tractor and farm machinery. As there were bridges involved, I can say that the velocity of the river was definitely affected but what about the width?

Well if there is, a slow velocity there would not be as much energy to make the width wider but instead to overcome friction from the bed load and banks. The widening of the river channel was caused by 3 types of erosion- Hydraulic power, Corrasion and Corrosion. Hydraulic power occurs due to the shear force and weight of the rivers load rubbing and creating friction against the rivers bed and banks.

This process gradually wears the bed and banks away and especially causes the removal of loose clay and sand. This process is even fiercer when the river is in flood because there is more gravitational potential energy due to more mass in the water. As there is more mass the water will travel faster, wasting more energy overcoming friction but by also rubbing harshly against the rivers bed and banks. Another big affecter in the widening of the river channel was Corrasion.

At times of high flow, boulders and pebbles are carried downstream by the force of the water. The pebbles and boulders will rub against each other and against the banks and bed of the river, which will also wear them away. Corrosion also occurs in the river as certain types of rock such as calcium carbonate CaCO 3 react with the water causing the rock to melt and dissipate out into the river water making it slightly acidic, this is bad for fish and some other aqueous animals.

If you look at my cross sections, you will notice the change in width as we moved from site 1 through to 4. You will also notice the increase in depth, which I shall explain next. The joining of tributaries to the main river at a confluence will also cause the river to widen as there is more energy made available.

As more and more tributaries join the main river, there will be an increase in water and other matter such as boulders and pebbles. There is more energy available for erosion as we move downstream due to the bed load becoming smaller and more rounded which shall cause the river to become more efficient and travel at a greater speed as it has more energy.