6.1 river processes

Transition from Ocean Systems to River Systems

In this segment of the course, there is a transition from discussing ocean systems to focusing on river systems, which are described as equally dynamic, if not more so, than ocean coastlines. The core idea is that rivers undergo constant change, encapsulated in the saying from the fifth century BCE by Heraclitus: "You cannot step into the same river twice." A more literal translation of Heraclitus's saying is: "Ever-newer waters flow on those who step into the same rivers." This notion serves as a foundation for understanding river dynamics.

Rivers are not only continuously changing themselves, but they also reshape the landscapes through which they flow. This critical relationship between rivers and land was not widely recognized in the Western world until the second half of the 19th century, particularly during the era when the United States embarked on extensive surveys to map the American West.

Lack of Historical Understanding

Historically, the understanding of how rivers interact with landscapes was limited, partly because the evidence was not always apparent in the familiar European landscapes. Many areas in Europe had their landscapes primarily formed by huge ice sheets that have now disappeared, leaving behind river systems that had not yet significantly altered the terrain. Unlike the European rivers, the survey crews exploring the American Southwest quickly realized that rivers there were instrumental in shaping the land, as the features and topography were directly influenced by river processes.

Common Misconceptions About River Valleys

The analysis of river valleys led to widespread misconceptions, such as the belief that the depth of a river valley can determine the age of a river. For instance, it was commonly thought that a very deep valley, like the Grand Canyon, indicated that the river was ancient. In contrast, shallower river valleys were perceived as more recent. This simplistic notion is incorrect; the depth of a valley is more indicative of the dynamic processes involved in river erosion and landscape formation rather than the river's age.

River System Profiles and Relative Sea Level Changes

Rivers respond dynamically to changes in relative sea level. Relative sea level can rise either because sea levels themselves increase or because the land experiences subsidence. From the river's viewpoint, these two processes result in similar outcomes: an increase in sediment deposition. As a river flows into a body of standing water, such as an ocean, its speed decreases, causing it to shed sediment, which then deposits, forming a new river profile. Therefore, when relative sea levels rise, rivers adjust by depositing sediment to maintain equilibrium with these changes.

Conversely, a relative sea level fall, whether due to the actual drop in sea levels or land uplifting, leads the river to begin eroding its banks and transport sediment further out to sea until a new equilibrium is established. This erosion creates deep river valleys, particularly evident in the American Southwest, as the land's tectonic uplift allows rivers to carve into the terrain.

Water Volume in River Systems Compared to Ocean Systems

Although rivers are integral parts of the Earth's water cycle, their volume is minimal compared to oceans. The overwhelming majority of the Earth's near-surface water rests in oceans, accounting for approximately 96.5% of it. In contrast, rivers contain only two ten-thousandths of a percent of the Earth's surface water at any moment. Specifically, this means that only two out of a million water drops exist within river systems at any given time. This raises the question of why rivers are significant despite their low volume.

Importance of Residence Time

Residence time plays a crucial role in the relevance of river systems. Residence time refers to the duration a water molecule remains within a particular part of the water cycle. The ocean's water remains in circulation for nearly 3200 years, while water molecules in rivers spend an average of only two weeks before reaching the ocean. This brief residence time leads to a high flux, or flow, within river systems.

Impacts on the Earth's Surface and Human Society

The high flux of rivers, despite their low volume, allows them to significantly shape the Earth's surface over time. This dynamic characteristic also plays a critical role in the interaction between rivers and human societies. Often, the beauty and dynamic nature of river systems can lead to potential conflicts. Rivers represent striking examples of dynamic equilibrium; changes in one part of a river can inadvertently affect the entire system, with many changes occurring gradually enough to be overlooked.

An example includes the dramatic alterations witnessed in the Mississippi River through St. Louis during the flood of 1993, which affected local residents significantly.

Sensitivity of Rivers to Flow Velocity

Rivers are characterized as remarkably dynamic systems, largely influenced by their sensitivity to flow velocity. The relationship between flow velocity and sediment transport is exponential; the ability of a river to carry sediment increases with the cube (3rd power) or the fourth power (4th power) of its velocity. For instance, if a river doubles its speed, it may be able to carry sediment 8 to 16 times more effectively. Moreover, changes in flow velocity not only affect the quantity of sediment transported but also the size of the sediment grains. A fast-moving river can even transport large boulders along its bed, while a slower river will struggle to transport much of its sediment, resulting in the deposition of finer particles.

Analogy with Student Walking Speeds

An analogy to illustrate the concept of flow speed and sediment transport involves comparing the average walking speeds of students on a campus. Research indicates an average walking speed of about 2.9 miles per hour, contrasted with ROTC students marching at approximately 3.4 miles per hour. This marginal difference in speed (about 0.5 miles per hour) could translate to significant differences in the volume and size of sediment transported by rivers at comparable speeds.

Erosion and Deposition in River Systems

The dynamics of river erosion and deposition depend on flow variations. For example, when flow speeds increase, such as during floods, rivers erode their banks. In contrast, sediment is deposited wherever flow slows down. This principle is observable in the formation of alluvial fans, which occur when streams flow rapidly down slopes, eroding and transporting sediments before depositing them as the water reaches flatter plains. In estuarine environments, such as deltas, rivers slow down as they enter standing waters, leading to sediment deposition that shapes the coastal landscape over time.

One pertinent point regarding floodwaters is that they carry substantial sediment loads. When floodwaters escape the confines of a channel, they slow down, resulting in sediment deposition across a wider area. The coarsest sediment generally deposits closest to the river, forming levees, which are elevated structures alongside the river channel that can contain rising waters. Consequently, levees are often amongst the highest features in a coastal floodplain.

These levees consist of coarser materials and influence vegetation growth, often supporting the growth of trees due to well-drained sediment.

Differential Erosion and Deposition

Within the same river system, one bank may be eroding while corresponding sediment builds up on the opposite side, forming point bars. This asymmetry arises from the river dynamics—water flows faster on the outer bank of a river bend, leading to erosion, while slower velocities on the inner banks promote sediment deposition.

Conclusion and Preview

In conclusion, dynamic processes govern river systems, influencing everything from erosion and sediment transport to landform evolution. The next segment will delve further into the various river patterns and their formation, continuing the discussion on the intricate interactions within river systems.