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Thumbnail: “Mountain Majesty” (CC-BY 2.0; Angela Sevin via Wikimedia Commons)
The Water Cycle
How many times a day do you take water for granted? Do you assume the tap will be flowing when you turn on your faucet? That the shower will turn on, the toilet will flush, and you’ll have water to cook your meals? Not only is water necessary for many of life’s functions, it is also a considerable geologic agent. Water can sculpt the landscape dramatically over time, by both carving canyons and depositing thick layers of sediment. Some of these processes are slow and result in landscapes worn down over time; others, such as floods, can be dramatically fast and dangerous.
What happens to water during a rainstorm? Imagine that you are outside in a parking lot with grassy areas nearby. Where does the water from the parking lot go? Much of it will flow across the surface and eventually join a stream. What happens to the rain in the grassy area? Much of it will infiltrate, or soak into the ground. Water is continually recycled through the atmosphere, to the land, and back to the oceans. This movement of water through the Earth System is referred to as the hydrologic (water) cycle (Figure 9.1). At Earth’s surface, this cycle, powered by the sun, operates easily since water can change form from liquid to gas (or water vapor) quickly. The majority of water is found in oceans, but freshwater can be found in lakes, rivers and trapped away in glaciers and ice sheets. Additional water resources are also found in the ground, and will be discussed in another chapter (Figure 9.2).
Those who study water, water resources, or the landforms made by water, may have many titles, including hydrologist, hydro-geologist, geomorphologist, or geochemist to name a few. Like many other geoscientists, working with other disciplines is common, with a heavy influence from both math and technology. Many are employed by universities where they teach and/or do research, and state and federal agencies, including geological surveys, like the California Geological Survey or United State Geological Survey (USGS). Additional career pathways are available in the private sector including in mining and natural resource extraction or in hazard mitigation and assessment. Many of these career options require a college degree and postgraduate work. If you are interested, talk to your geology instructor for advice. We recommend completing as many math and science courses as possible. Also, visit National Parks, CA State Parks, museums, gem & mineral shows, or join a local rock and mineral club. Typically, natural history museums will have wonderful displays of rocks, including those from your local region. Here in California, there are a number of large collections, including the San Diego Natural History Museum, Natural History Museum of Los Angeles County, Santa Barbara Museum of Natural History, and Kimball Natural History Museum. Many colleges and universities also have their own collections/museums.
Stream Drainage Basins and Patterns
The drainage basin of a stream includes all the land that is drained by one stream and all of its tributaries. Find out more about the drainage basin or watershed you live in by visiting the Environmental Protection Agency’s How’s My Waterway?.
The higher areas that separate drainage basins are called drainage divides. For North America, the Continental Divide in the Rocky Mountains separates water that drains west to the Pacific Ocean from water that drains east to the Gulf of Mexico or Atlantic Ocean (Figure 9.3).
As water flows over rock, it is influenced by it. Water follows the path of least resistance; this means it will weather and erode softer rock first, rather than more resistant rock. This can result in characteristic patterns of drainage (Figure 9.4). Some of the more common drainage patterns include:
Dendritic: this drainage pattern indicates uniformly resistant bedrock that often includes horizontally layered sedimentary rocks. Since all the rock is uniform, the water is not attracted to any one area, and spreads out in a branching pattern, like the branches of a tree.
Trellis: this drainage pattern indicates alternating resistant and non- resistant bedrock that has been deformed (folded) into parallel ridges and valleys. The water erodes valleys in the softer rock, and appears much like a rose climbing on a trellis in a garden.
Radial: this drainage pattern forms as streams flow away from a central high point, such as a volcano, resembling the spokes in a wheel.
Rectangular: this drainage pattern forms in areas in which rock has been fractured by jointing or faulting which created weakened zones in the . Streams erode the weakened, less resistant rock and create a network of channels that make right-angle bends as they follow the intersecting fracture pattern. This pattern will often look like rectangles or squares.
Deranged: this drainage pattern does not follow the rules. It consists of a random pattern of stream channels characterized by irregularity. It indicates that the drainage developed recently and has not had time to form one of the other drainage patterns yet.
How Do Streams Move Sediment?
The running water in a stream will erode and move material within the stream channel. The transported material includes 1) the dissolved load, dissolved substances taken into solution during chemical weathering, 2) the suspended load, tiny silt and clay particles that are kept in suspension by the water’s flow, and 3) the bed load, visible sand and gravel-sized sediments that typically travel along the stream bed. Here, grains move either via hopping (saltation), rolling, or sliding (traction) (Figure 9.5). The measure of the total sediment a stream can carry is called stream capacity.
Stream competence reflects the ability of a stream to transport a particular size of particle (e.g., boulder, pebble, etc.). An increased velocity of water flow increases stream competence, whereas a decreased velocity of flow decreases stream competence (Figure 9.6).
While the dissolved, suspended, and bed loads may travel long distances, they will eventually settle out and be deposited. These stream deposited sediments, called alluvium, are typically deposited during flood events. This is because to more effectively transport sediment, a stream needs energy. This energy is mostly a function of the amount of water and its velocity. A fast-moving stream is more capable of carrying much more and larger sediment. As a stream loses energy, it will slow down, which is why deposition occurs.
Under normal conditions, water will remain in a stream channel. When the amount of water in a stream exceeds its banks, the water that spills out of the channel will rapidly decrease in velocity. A decrease in velocity results in the deposition of the larger sandy material the river carries along the channel margins. These ridges of sandy alluvium are natural levees (Figure 9.7). As numerous flooding events occur, these ridges build up under repeated deposition. These levees are part of a larger landform known as a floodplain. A floodplain is the relatively flat land adjacent to the stream that is subject to flooding during times of high discharge.
How Do Streams Erode?
Rivers can erode laterally (sideways), vertically (downcutting) or back into the upland area (headward erosion), all of which result in the river becoming wider, deeper, and longer (Figure 9.8). Headward erosion occurs when the river erodes in an upstream direction, lengthening the river valley. Downcutting, or vertical erosion, occurs as the river erodes downwards, deepening the river channel. Lateral erosion occurs along the edges (banks) of the river, widening the river valley.
Stream gradient refers to the slope (rise over run) of the stream’s channel. It is the vertical drop of the stream over a horizontal distance. Gradient can be calculated by using the following equation: Gradient = (change in elevation) / distance. Stream gradients tend to be higher at a stream’s source, or the headwaters, and lower at the mouth.
Examine Figure 9.9. We want to determine the gradient from A to B. The elevation of the stream at A is 980’, and the elevation of the stream at B is 920’. Referencing the scale bar, the distance from A to B is about 2 miles. Using the equation above: Gradient = (980’-920’) / 2 miles, or 30 feet/mile (read 30 feet per mile).
How Is Streamflow Measured?
The United States Geological Survey (USGS) is one of the major organizations tasked with measuring streamflow here in the US. More information about how they do that is available via their Streamflow Measurement website.
In general, discharge measures stream flow at a given time and location. It is a measurement of the volume of water passing a particular point during a period of time. Discharge can be calculated by using the following equation, Discharge = Area * Velocity. Area is determined by multiplying the width by the depth of the stream channel. Velocity of the water is typically measured in units of feet per second or meters per second, while discharge has units of cubic feet per second (cfs) or cubic meters per second (cfm). Discharge increases downstream in most rivers, as tributaries join the main channel and add water.
Sediment load also changes from headwaters to mouth. At the headwaters, tributaries quickly carry the load downstream, combining with loads from other tributaries. The sediment is eventually deposited when the stream reaches base level. Occasionally during this process of transporting material downstream, the sediment load is large enough that the water is not capable of transporting it, so deposition occurs. If a stream becomes overloaded with sediment, a braided stream may develop. Typically, these streams have a network of intersecting channels that resembles braided hair with sand and gravel bars common. Braided streams are common in sediment-abundant areas, near glaciers or in arid and semiarid regions with high erosion rates.
Streams may also be meandering, with broadly bending meanders that resemble “S”-shaped curves. Water will travel the fastest on the outside of a bend. This higher velocity leads to more erosion on the outside bend, forming a cut bank. Erosion at the cut bank is offset by deposition on the opposite bank of the stream, where the water moves slower and allows sediment to settle out and deposit. These areas of deposition are called point bars. As meander bends become more complicated, or sinuous, they may intersect to form a cutoff, which shortens the stream’s path. After cutoff the abandoned meander loop becomes a crescent-shaped oxbow lake (Figure 9.10 and 9.11).
Straight streams, in which channels remain nearly straight, are uncommon. They can form naturally due to a linear zone of weakness in the underlying rock or can be human constructed, in an effort at flood control, much like what has been done with the Los Angeles River (Figure 9.12).
Flood Stage and Flooding
Flooding is a common and a serious problem on and along our nation’s waterways. Flood stage is reached when the water level in a stream overflows its banks. Floodplains are popular sites for development but are best left for playgrounds, golf courses, and the like.
Have you ever heard someone say, “that flood was a 1 in 100-year flood”? What does this mean? Does it mean that a flood will only occur every 100 years, and that we are safe the other 99? The short answer is No; on average, we can expect a flood of this size or greater to occur within any 100-year period. However, we cannot predict whether it will occur in any particular year, only that each year has a 1 in 100 (1%) chance of occurring in any year. The timing between these major flooding events, or any major geologic event (drought, volcanic eruption, earthquake, tsunami, etc.), is referred to as a recurrence interval. This is the average time period within which a given flood event will be equaled or exceeded once.
In order to better understand stream behavior, the USGS has installed thousands of stream gauges throughout the country, locations with a permanent water level indicator and recorder. Data from these stations can be used to make flood frequency curves, which are useful in making flood control decisions.
Figure 9.1: “The Water Cycle” (Public Domain; Howard Perlman and John Evans, USGS)
Figure 9.2: “Where is Earth’s Water?” (Public Domain; USGS Water Science School)
Figure 9.3: “Continental Divide” (CC-BY 4.0; Chloe Branciforte via Google Earth)
Figure 9.4: Derivative of “Drainage Patterns” (CC-BY-SA 3.0; Corey Parson via LibreTexts)
Figure 9.5: “Stream Load” (CC-BY 4.0; Emily Haddad)
Figure 9.6: Derivative of “Hjulströms diagram en” (CC-BY-SA 3.0; Karrock via Wikimedia Commons) by Chloe Branciforte
Figure 9.7: Derivative of “Levees” (CC-BY-SA 3.0; Julie Sandeen via LibreTexts) by Chloe Branciforte
Figure 9.8: “Stream Erosion” (CC-BY 4.0; Emily Haddad, own work)
Figure 9.9: Derivative of “Gradient Calculation” (CC-BY-SA 3.0; Randa Harris via LibreTexts) by Chloe Branciforte
Figure 9.10: Derivative of “Meandering River” (CC-BY-SA 2.5; Maksim via Wikimedia Commons) by Chloe Branciforte
Figure 9.11: “Klamath River” (CC-BY 4.0; Chloe Branciforte via Google Earth)
Figure 9.12: “LA River” (CC-BY-SA 3.0; Joe Mabel via Wikimedia Commons)
Figure 9.13: “Inundation of the State Capitol, City of Sacramento, 1862” (Public Domain; A. Rosenfield via Calisphere)