# Planet Earth/5g. Earth’s Rivers
## Exploring Earth’s Rivers

On a wet and muddy April day, John Wesley Powell and his Battery “F” artillery faced a massive army of confederate soldiers marching toward them in Southern Tennessee. As he gave the command to fire the cannons to his company, a lead bullet sliced through his right arm. The cannons blasted around him and the mangled arm gushed blood, as the horror of war fell upon them all. Born in New York, Powell’s family moved west to northern Illinois, where he discovered his love of rivers. His restless nature led him on a series of adventures to travel the great rivers of the region by boat. First in 1855, hiking across the state of Wisconsin, then a great adventure in 1856 to follow the Mississippi River from St. Anthony, Minnesota, until it reaches the Gulf of Mexico, a journey by boat of about 2,300 miles (3,700 kilometers), and in 1857 he followed the Ohio River and then Mississippi River from Pittsburgh to St. Louis, a journey of about 1,500 miles. Between these adventures in exploring, he attended classes at Illinois College (today’s Wheaton College), and taught small science classes at the college, lecturing on his river adventures and map making. In 1861, with his brother Walter, Powell enlisted in the Union Army with the outbreak of the American Civil War. The autumn before being sent into battle, John Wesley Powell married his first love, Emma Dean. The two brothers John and Walter were placed in the same artillery unit and on that fateful day and faced an army together on the western banks of the Tennessee River. At the first morning light, a massive assembly of confederate soldiers advanced on the encamped Union Army. By sundown, the two brothers would be changed forever. John Wesley Powell would lose his right arm that day, in a two-day battle that became known as the Battle of Shiloh.
His right arm was amputated following the battle, while his brother Walter would lose his mind. Uninjured, Walter continued to serve in the artillery, firing cannon balls into the living flesh of advancing soldiers. In 1864, Walter was captured by the Confederate forces near Atlanta, and although attempted to escape from the war prison, he was recaptured, nearly starved to death, and finally released as part of a prisoner exchange. With the victory of the Union, the two brothers returned back to Illinois, one without an arm, the other suffering of the mental horrors of war and life in a prison camp.
The country had also changed during those war-torn years, the adventures on the rivers and tributaries of the Mississippi River that John Wesley Powell had done before seemed small, as new efforts were pushing people westward in search of a new life. The completion of the Transcontinental Railroad in 1868, drastically shorted the distance to travel from coast to coast overland, a journey that could take over a year to make by ship, now people could journey in the comfort of a rail car in weeks, with a simple ticket purchased at a rail station. Much of the American West remained completely unexplored, while John Frémont mapped much of the west during the onset of the Mexican-American War, there were still large regions not well explored and mapped, particularly the complex canyons of the Green River and Colorado River in eastern Utah and Arizona. In 1859, prior to the civil war U.S. Government Surveyors, led by John N. Macomb, attempted to locate the confluence of the two rivers, but had failed. Maps of the United States still had large unexplored regions in the American West, particularly in the deserts of the Southwestern States of Utah, Colorado, New Mexico and Arizona.
After the war, John Wesley Powell returned to teaching in Illinois, but traveled to Colorado with his wife and some students to collect natural history specimens for the museum at the college in 1867 and 1868, and during these trips west, Powell hatched a daring plan to run the length of the Colorado River drainage, as he had the Mississippi River. Traveling by rail to Green River, Wyoming, he could access the large Green River, which snaked through deep canyons into Colorado, Utah, Arizona and parts unknown. With funding from the United States Government, on May 24, 1869, he and his brother, Walter, and a crew of 8 other men set out on three boats down the river. Passage down the Green and Colorado Rivers was not as simple as the journey down the Mississippi, as they had to pass through dangerous rapids and roaring white waters in wooden boats.
Early in their journey one boat was bashed to pieces against rocks in the rapid currents, and one of their crew abandon the journey near Ouray, Utah, the others pressed on. In Arizona they entered into the Grand Canyon region, as the Colorado River sliced into the largest canyon on Earth. Along the journey Powell surveyed the river, mapping the region, and frequently hiking over the landscape, measuring the height of mountains using a barometer, using a sexton to measure their latitude and chronometer to measure their longitude, as they traveled through the wild country, describing the rocks, plants, animals, and the people that lived along the river. When the river rapids got even worse, and it appeared that they could not travel safely any further, 3 members of the team attempted to hike out of the canyons, and where later found dead. The remaining crew continued on, down the river. As veterans of the Civil War, they were not daunted by their journey into the unknown. Feared dead, newspapers ran stories of their unsuccessful trip, but on August 30, 1869, the remaining crew, including both John Wesley and his brother Walter Powell had arrived in Yuma, Arizona. Powell had personally seen more section of river than anyone alive, the two major river drainages of North America. Two rivers that contrast in their very nature; the low winding path of the gentle Mississippi River, and the wild roaring white waters of the Colorado River.

## The Nature of Rivers

What is the science behind the nature of rivers? How do they flow across the land? How do they carve canyons that defy the imagination? Or bring silt and mud to their flooded banks? Rivers are the earthly passage of freshwater. Water from rain and snow on the land that trickles back to the oceans. The sinuous path defining where one region can support farms and crops, and cities, while other regions are dry deserts, with rivers cut into steep canyons.
The success of the river trip catapulted John Wesley Powell into national fame, although he returned to the river in 1871 and 1872 on a second expedition. This time taking with him photographic equipment to document the wonders that they saw. In 1875 he published his notes and descriptions, which became his bestselling book The Exploration of the Colorado River and Its Canyons. The success of his exploration of the Colorado River resulted in his appointment in Washington D.C. to serve as the director of Bureau of Ethnology at the Smithsonian Institution to help preserve Native American culture and languages, and the United States Geological Survey to oversee the survey of the American West. Powell cautioned the government about the importance of rivers from understanding the flooding along the banks of the wet Mississippi River to water scarcity issues in dry deserts along the canyons of the Colorado River. Not all since then have yielded to his recommendations, as rivers have been dammed and drained. The Hoover Dam constructed during the Great Depression in the 1930s resulted in the reservoir of Lake Mead in Nevada, and the Glen Canyon Dam constructed in the 1960s resulted in the reservoir of Lake Powell in southern Utah. The Green River also received a dam, the Flaming Gorge Dam in 1964, generating a large reservoir of freshwater in northeastern Utah, and Wyoming. These man-made restrictions to the flow of rivers were a result of the desire to retain water for agriculture and generate electricity to serve cities and towns that have rapidly grown in the arid deserts in the American Southwest.
The nature of rivers, including streams, creeks, and other tributaries, can be simply defined as bodies of moving water through a channel. Rivers transport eroded portions of the solid Earth, and deposit these loose sediments far from their original source. Geologists refer to this process of erosion and deposition caused by rivers, as fluvial processes. Rivers form within specific areas of drainage, where they form a catchment. A catchment is defined by high topography, often mountainous terrain, which divides the flow of water. These high topographic boundaries are referred to as drainage divides or watersheds, as water is shed off these slopes and into the drainage basin. Rivers flow toward the lowest topography of the landscape, incising channels that form a complex tributary system or network across the landscape. Each stream segment or link of a river system can be classified by a stream order. For example, during Powell’s river trip down the Green River, many other smaller rivers drained into the Green River, including the Yampa (or Bear) River, and the White River, each time accumulating the additional flow of those rivers. The Colorado River was once defined as the junction of the Green River and the Grand River, but since this naming convention would mean that the Colorado River did not flow in the State of Colorado, the name Grand was dropped and instead geographers extended the name Colorado upriver. Geomorphologists, who study the shape of river systems, define these names based on stream ordering, with 1st order streams being tiny tributaries, while higher orders like the 10th order, are composed of the sum of 10 tributaries, and are much larger rivers within the drainage basin.
Rivers flow downslope over various gradients, with steep gradients in the mountains, and gentle gradients in the lowlands. The average gradient of the Colorado River is around 5 meters per kilometer, whereas the average gradient of the Mississippi River is only 0.01 meters per kilometer. The gradient is much higher along the Colorado River, with sections of the river that fall 12 meters per kilometer, producing the whitewater rapids that make the river famous. The gradient or slope of the river controls the overall river’s behavior as it moves across the landscape. The velocity or speed at which the water flows is also related to the gradient, but may not be how you might think. Rivers with a steeper gradient like the Colorado River exhibit slower water velocity than large low gradient rivers like the Mississippi River. The water in the Colorado River is slowed down by the rocks and other obstacles to the flow of water that actually slows the speed of the water, while the Mississippi River water is less impeded by rocks and other large obstacles in the river. The velocity of a river is measured in two different ways, one by measuring the length of time a piece of wood or other floating item passes a measured distance along the river and the other accurate way is to use a flow meter, which measures the speed of the water by its ability to turn a propeller.

## Measuring River Discharge

One of the most important measurements of a river’s flow is Discharge (Q). Discharge is the total amount of water passing through a point along the river at a specific interval of time. To measure discharge, the rivers velocity, width and depth must be known. Q = V x W x D. Discharge in rivers is measured by a river gauge station. A gauge station is a concrete lined section of river that restricts the flow of the river within a specific width. Depth is measured using a ruler scale on the side of the flowing river, and velocity using a flow meter. Discharge is often monitored on various sections of rivers that are prone to spring flooding, as when discharge rises quickly following a storm, flood warnings can be issued to people living down river. The average discharge of rivers will increase with their increasing order, as more water enters from the surrounding tributaries that drain into the main river channel. The Mississippi River has an average discharge around 200,000 to 700,000 cubic feet per second entering the Atlantic Ocean. The Colorado River during the time of Powell’s trip had an average discharge around 22,500 cubic feet per second entering the Pacific Ocean, today almost no water from the Colorado River reaches the Pacific Ocean, as most water is lost due to its evaporation and use upstream, held back by the many dams along the river. In a natural state, rivers will increase the amount of discharge downstream, with the largest amount of water flowing into the oceans, forming river deltas or estuaries.
River deltas form when sediment carried by the river is deposited as the carry capacity of the river decreases when it reaches the ocean, this leads to a buildup of sediment deposition which forms a complex of mud banks that splits the river into many channels as it navigates access this region of sediment deposition. River estuaries form when sea level rises and ocean processes, such as tides and waves push upriver, and flood river valleys. Estuaries are known for the brackish water they contain, a mix of salty ocean water and freshwater.

## Braided versus Meandering Rivers

Geologists define two types of river systems. Braided rivers, which form braided channels interlaced between complex sand bars, and Meandering rivers, which form sinuous channels that snake across the landscape. Braided rivers are limited to regions where there is a large sediment load carried by the river, dominated by spring runoff, and tend to be located near mountainous regions fed by glaciers. The flow of the water will follow a complex braided pattern between sand bars, which channel the flow of water between them. These sand bars are reworked, especially during spring floods, but may become stable for shorter intervals of time. Braided river systems are sediment heavy, and experiments had shown they form when rocks and sediment are loosely held. Braided river systems are thought to have been the dominant form of river systems before the evolution of terrestrial plants on the surface of Earth, since the roots of plants hold sediments together, and establish stronger river banks. This prevents braided rivers from forming.
Meandering River systems are the dominant type of rivers today on Earth’s surface. They form sinuous undulating paths across the land. These meanders are formed by the balance between the velocity of the flowing water and increasing effect it has on erosion against the banks of a river channel. Rivers never flow straight from one location to the next, and this is because the distribution of slight differences in the velocity of flowing water inside the river. Within the river, water flow tends to slow to one side of the river as it moves across the landscape, while the other side flows faster. This higher velocity to shift toward one side of the river is a result of the frictional forces acting against the flowing water. One side of the river will flow relatively faster and begin to erode the edges of the river bank on that side of the channel. This region of the river bank is called the cut bank of the river, the side of the river that the flow of water cuts into the edges of the river channel. On the opposite side of the river, the velocity will be less, and a point bar will form, which is where sand and other sediment transported by the river will begin to accumulate forming a point bar, a thick layer of sand and sediment. The highest velocity within the flow of the river will be off center below the cut bank, and within this zone lag deposits will form, which are large rocks which are near the limit of what the river water can carry downstream. While the point bar will accumulate smaller sediments, due to the less capacity for the river to carry larger rocks and coarse sediments.
Over time the river will continue to cut into one side of the bank on the high velocity side, while the other side of the river will grow with the deposition of sand and other sediments, on the low velocity side. The river will alternate downstream with the cut bank and point bar on differing sides of the river. Eventually when the cut bank continues to be eroded the river will increase the bend, until contacting another cut bank along the course of the river. This bypass forms an oxbox lake, an isolated section of river that has been bypassed by the main river channel.
Experiments conducted in large sandboxes, tilted at different gradients or slopes demonstrate that meandering rivers require sediments to be stabilized, either by roots or lithification. If the sediments are loose, like sand, the river pattern will result in a braided river system. With the advent of terrestrial plants most fluvial systems on Earth will meander, but not all rivers exhibit meandering patterns, especially when the river is highly seasonal and flows across loose sediments like sand.
Throughout their history rivers on Earth have moved enormous amounts of sediment from the continents and toward the oceans, through a process of denudation. Denudation are the processes that cause the wearing away of the Earth’s surface by moving water. These sediments accumulate in drainage basins and along coastlines of continents, where they are deposited. Rivers tend to move sediments also horizontally across a drainage basin as they cut into one side of the river bank, and redeposit sediments on the opposite side on the point bar, but slightly more downstream. This deposition of sediments results in cross-bedding which is tilted downward toward the channel axis. Geologists can measure these cross-bedding features to determine the changing directions of flow of river systems over time. Rivers also cut down through the landscape, leaving terrace deposits of coarse river pebbles at different elevations during this process of eroding deeper into a canyon. These terrace deposits, if dated, can yield a history of canyon or valley incision for the river system.
Rivers are held along the banks by natural levees. A natural levee is a low ridge of sediment and/or organic materials, such as wood that is deposited alongside a river by water during their peak seasonal flow. This deposit of sediment forms a low ridge that acts to retain the river channel within the main channel system during seasonal runoff and at annual high discharge levels. However, during infrequent flooding events these natural levees can be breached, resulting in floodwaters covering the larger flood plains beyond them. Flood plains are the low topographic lands adjacent to rivers that experience infrequent flooding during periods of extremely high discharge. Because of their close proximity to water, flood plains are areas that are desirable for the construction of homes and buildings near irrigated lands. Unfortunately, they are where flooding can cause considerable damage, when rivers overflow their natural protective levees.

## Flood Frequency Analysis

In 1928, Emil Julius Gumbel Professor of Math at the University of Heidelberg, published research in a book that would result in a death warrant being placed on his head. He was not a geologist nor researcher on rivers and floods at that time, but on the mathematical statistics of extreme events. As early as 1919, political murders were being committed by members of the Freikorps, a paramilitary group that would later give rise to the Nazi Party’s Sturmabteilung, or “brown shirts.” Gumbel documented the rise of these political murders in the 1920s, by tracking the statistics of recorded deaths across the country, and noting the political motivations of these deaths. Murder is an extreme event, and statistically they should be of low frequency; a burglary gone bad, murder resulting from domestic violence, a murder suicide, or even mass shootings, all extreme events. But Gumbel wanted to know if these events were a result of a rising political paramilitary that was ordering these killings secretly. In 1928, he published his second book on the topic, demonstrated the rising murder rates were in fact a result of political killings by a secret paramilitary force working in Germany. With concern he too would be murdered by these forces, he fled to France and later during the war in 1940, to the United States, where he continued his mathematical work. After World War II, and the defeat of the Nazi Party in Germany, he applied his mathematical work to other extreme events, such as floods.
Both floods and murders can be described by their recurrence interval. The recurrence interval is the length of time between two extreme events. The more extreme the event, the longer the recurrence interval will be. A 100-year flood, would represent a flood that reaches the highest maximum discharge only once every 100 years, while a 10-year flood, would represent a flood that reaches its maximum discharge once every 10 years. Recurrence intervals are often plotted with their corresponding discharge (Q), forming an increase sloped regression line; the longer the recurrence interval the higher the total discharge (Q) will be.
Whether it is murders or floods, recurrence intervals can tell you a lot about a particular place. For example, if the recurrence interval of a murder is once a year in a particular town, it is not going to be as safe as a place as a town that only experiences a murder every 100 years. The same is true with floods, you would not want to live in a house that would flood every 5 years, but a house in an area that floods every 250 years would be at less risk of flooding, and a safer place to live. It should be noted that these extreme events are probabilities, like rolling dice with different number of sides. The probability that a 100-year flood to occur would be the same as rolling a 100-sided dice, and having the number 42 appear. Each year would be a new roll of the dice. If the dice the following year also was 42, meaning that two 100-year floods happened in two consecutive years, the probability of this would be a multiple factor greater. For example, the probability that 100-sided dice rolls 42 is 0.01 (or 1:100). The probability that the second roll of 100-sided dice is 42 is 0.01 (or 1:100). However, the probability that the first roll is rolls 42 AND the second roll rolls 42 is equal to 0.01 x 0.01 = 0.0001 or (1:1,000). The probability that the first roll is 42 AND the second roll is 42 AND a third roll is 42 would be 0.01 x 0.01 x 0.01 = 0.000001 or (1:100,000). If you continue to roll 42, you would begin to question that the dice really has 100 sides, and might be fixed. In the case of murders in 1920s Germany, Gumbel documented the uncanny rise of murders with the rise of the Nazi Party, and sounded alarms about a major political coverup using math. But with rivers, how do we measure floods, especially those extreme flooding events that rarely occur and may not be documented?
Real time measurements of a river’s discharge (Q) are recorded at gauge stations along the course of the river in cubic meters per second (m$^3$/s) or cubic feet per second (ft$^3$/s), by measuring the width, depth and velocity of the river’s flow. This data is sorted by peak discharge and the date of that peak discharge. Each date’s discharge is numerically ranked from the greatest discharge to the smallest discharge. The return period for each discharge is calculated by the inverse probability that the flood will be exceeded on that given date of a given rank. If the return period is 0.01 (1:100) for a given year, then the reoccurrence of a flow of this discharge will be 100-years. This method of analysis is named the Gumbel distribution; named after Gumbel who developed the mathematics in 1935, and later applied it in the study of floods in 1941. Gumbel’s statistical analysis focuses on the probabilities of extreme events, from catastrophic floods to the rise of authoritarian regimes. Today, Gumbel’s studies of extreme events are important in understanding the effects of climate change on Earth, particularly how changes in the atmosphere might affect the flow of rivers across Earth’s surface.

## The Importance of Freshwater

In 1973, at the age of 16, Mae Jemison left her home in Chicago to attend Sandford University in California with a scholarship. Her life-long hero was the famous ballerina Judith Jamison, and although she wanted to be a fulltime dancer like her hero, she declared her academic major in African-American studies, a new program born out of the civil rights movement during the decade before. Jemison was also interested in engineering and science and was also fan of the original Star Trek television show. As an attractive brilliant young woman, with African heritage she shyly stayed in the back of the science and engineering classes when she enrolled in them. The majority of her classmates were white men, and she felt oddly out of place being a young woman and African-American. On the first day of her Fluid Dynamics class, she hid, as she typically did in these science classes, at the back of the classroom, but her professor, noticing her there assigned her to work with a group of classmates in the front of the class. Through the experience in the class Jemison developed a love of science and engineering, and after graduating she went on toward a career as a medical doctor in New York. After medical school, Jemison served as a doctor in Southeast Asia and West Africa for several years, before returning to the United States. Her interest in space travel compelled her one day to call up the Johnson Space Center and ask them if she could apply to be an astronaut. They sent her an application, and she applied. Physically fit, with her years as a professional dancer, degrees in science and engineering, as well as her experience as a medical doctor, made her an ideal candidate for an astronaut, but on January 28, 1986, the NASA Space Shuttle challenger exploded on take-off killing all five astronauts on board. Despite the disaster and with future trips to space canceled, Jemison was brought in by NASA in 1987, one among 2,000 fellow applicants and trained to be an astronaut for the 1992 Space Shuttle Endeavour mission.
Her role would be to study the effects of space travel on health, particularly studying the use of intravenous fluids in space and low gravity. Finally, she achieved her dream. High above Earth Jemison looked down through the space craft’s windows and saw the fragile nature of the place we call home. Seeing the entirety of Earth completely surrounded by the darkness of space made her realize the importance of safeguarding Earth’s supply of water. On her return to Earth, Jemison left NASA and became a professor of Environmental Studies at Dartmouth College. Her experiences as a medical doctor and as an astronaut compelled her to realize the importance of clean fresh water. She started science camps to promote, not only the exploration of outer space, but a new generation of scientists to tackle the problems facing Earth’s freshwater supply.