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California Rivers and Streams: The Conflict Between Fluvial Process and Land Use
California Rivers and Streams provides a clear and informative overview of the physical and biological processes that shape California's rivers and watersheds. Jeffrey Mount introduces relevant basic principles of hydrology and geomorphology and applies them to an understanding of the differences in character of the state's many rivers. He then builds on this foundation by evaluating the impact on waterways of different land use practices―logging, mining, agriculture, flood control, urbanization, and water supply development.
Water may be one of California's most valuable resources, but it is far from being one we control. In spite of channels, levees, lines and dams, the state's rivers still frequently flood, with devastating results. Almost all the rivers in California are dammed or diverted; with the booming population, there will be pressure for more intervention.
Mount argues that Californians know little about how their rivers work and, more importantly, how and why land-use practices impact rivers. The forceful reconfiguration and redistribution of the rivers has already brought the state to a critical crossroads. California Rivers and Streams forces us to reevaluate our use of the state's rivers and offers a foundation for participating in the heated debates about their future.
Water may be one of California's most valuable resources, but it is far from being one we control. In spite of channels, levees, lines and dams, the state's rivers still frequently flood, with devastating results. Almost all the rivers in California are dammed or diverted; with the booming population, there will be pressure for more intervention.
Mount argues that Californians know little about how their rivers work and, more importantly, how and why land-use practices impact rivers. The forceful reconfiguration and redistribution of the rivers has already brought the state to a critical crossroads. California Rivers and Streams forces us to reevaluate our use of the state's rivers and offers a foundation for participating in the heated debates about their future.
378 pages, Paperback
First published November 8, 1995
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October 7, 2024
I didn't realize I knew so little about rivers. There were four things about this book that I appreciated:
1. The first half of the book covers the dynamics of rivers. Everything from how they shape the landscape around them via erosion and sediment deposition, to how water reaches them (via overland flow, groundwater, etc), to how they get their shapes (meandering, braided, shallow, deep, etc), to how they are influenced by climate. The book opened my eyes to how many of the landscapes I have seen, especially in California, have been shaped by rivers. One thing that was especially interesting to me was how sediment is carried by rivers. Rivers have two qualities, "capacity" and "competence" - capacity is how much sediment load a river can carry, and competence is the maximum size of the individual pieces of sediment that they can carry. If a river's competence decreases, that will cause it to deposit sediment. Competence is controlled by variables such as river gradient and velocity. So, for example, as a river flows down a mountain valley, it will have high gradient/be fast and pick up a lot of gravel-sized sediment. When the river reaches the base of the mountain, it will slow down/spread out, reducing its competence and causing it to deposit the gravel-sized sediment. Therefore you will notice that at the bases of mountains, rivers will typically have large gravel bars. As the river flows towards the see, typical its gradient/velocity get smaller and smaller, causing finer and finer sediment to be deposited. The mouth of the river will have the finest sediments, basically clays and fine silts. Another interesting discussion in the book is about groundwater. Groundwater has always seemed mysterious to me but the book explained how it works pretty well. Under the land surface, there is typically an are called the "unsaturated zone" that is dry. But at a certain level called the water table, the earth becomes saturated with water. Groundwater moves very slowly, allowing the accumulation of gradients in the height of the water table. For example, it's possible that under a hill, the water table bulges up. These gradients in water table height cause groundwater to flow. For example, the high water table under the hill will slowly flow down and away from the hill. The height of a river during "base flow" (aka when there is no storm run-off or snow melt influencing flow) is typically at the height of the local water table. So for example there are rivers in California that flow all year despite the lack of summer rain because the groundwater in the neighboring mountains gives them a steady supply of water. However, in areas where river channels have been deepened/scoured (typically by human modification), the river might actually flow below the level of the water table, especially during droughts. This is really bad because it will cause groundwater to drain into the river, lowering the entire water table of the region. Alternatively, humans often pump groundwater out of the ground, causing the water table to lower. This can greatly reduce the flow of rivers as their water gets sucked into the ground to replenish the water table.
2. The impact of rivers on human life. The book discussed how rivers' shapes are determined by "bankfull stage", i.e. the water level at which a river reaches the top of its channel. This stage happened on average every 1-2 years, so it's basically the most frequent "significant" flow that the river experiences and therefore has the most influence in shaping the river. However, unmodified rivers experience discharge rates according to an approximately log-normal distribution apparently, with approximate recurrence rates such as the so-called "100-year flood". Although rare, discharge rates that cause rivers to overflow their banks are inevitable, and the flooded river will expand to its floodplain, where the much large "hydraulic radius" will cause the river to slow down and deposit sediment. Basically, you should think of the floodplain as being an integral part of the river, which it uses to store excess sediment. Almost any flat geographical area was or is a floodplain. And humans really shouldn't live on them without strong flood protections, as the devastation recently wrought by Hurricane Helene in North Carolina shows.
3. The impact of humans on rivers. The second half of the book goes into great detail about how human modifications affect rivers. Logging, for example, especially "tractor yarding", greatly increases the runoff and sediment load of rivers not because of the cleared trees but because of the compacted roads that are created in order to reach trees. Urbanization reduced sediment load but greatly shortens the "hydrograph response" of a river (how a storm will affect the flow of a river - peak flow will be much higher and much sooner) due to concrete/asphalt urban surfaces and how we modify our urban rivers by straightening them and lining their beds with concrete. The construction of levees often has perverse upstream and downstream effects - the narrowing of the river channel can cause the river to get "backed up" upstream of the levee, causing flooding. Meanwhile, during floods, the faster, higher flow of the river in the levee will also cause flooding downstream of the levee. This creates a race to levee the entire river, which can have dangerous consequences during especially large floods when the levee is at, near, or above capacity, which can cause levee failure. Humans also mine river channels, typically for gravel. The mining operations typically greatly alter the river channels both upstream and downstream of their operations as the river channel struggles to adjust to the huge depletion of sediment at the location of the operation. The biggest human impact of all, though, must be dams. Dams complete alter dammed rivers. First of all, dams capture basically all sediment, so rivers downstream of the dam are "sediment hungry" - they scour their beds for sediment, incising into the landscape (which can cause water table drainage) typically until the river becomes "armored" (its channel is full of huge rocks that the river is too weak to move). On the other hand, as tributaries enter the main channel, the reduced flow of the main channel means that finer-than-usual sediments are deposited in the channel, causing it to fill up and shrink. Both of these changes are devastating for salmon and related fish which like to lay their eggs in the gaps between pieces of gravel, which often either get swept away or filled with fine silt. By constricting the flow of the river, dams have converted braided rivers into meandering rivers.
4. The vast scale of water engineering in California. Before reading this book, I was so oblivious to the enormous extent to which California has engineered its water supplies. This was necessary because most of California's rain falls in the remote north, and most of the population land a lot of the agriculture happens in the south. We have dammed almost every major river in California. We have constructed massive aqueducts to transport that water, such as the Bay Area Aqueduct and the California Aqueduct. There are massive pumping stations located throughout the state that pump those waters over our mountains and into our population centers. Our flood control structures are also very interesting, especially the Yuba Bypass west of Sacramento, which is a huge area devoted to be an occasional floodplain for the Sacramento River which spares West Sacramento from being flooded. Lastly, the Sacramento-San Joaquin Delta is a hugely underappreciated part of the state. Much of our freshwater flows through the delta before being pumped into canals and aqueducts for human use. The delta is very vulnerable because it lies at sea level and is exposed to the salt waters of the San Francisco Bay, so increases in sea level will cause salt water to intrude into the delta.
There were many other interesting discussions in this book, from the specifics of California climate (such as how our cold-wet, cold-dry, warm-wet, and warm-dry storms form) to the ongoing impact of hydraulic mining in the 1800s, to the expected impacts of global warming/climate change. Overall, it was a really fascinating overview of an aspect of our environment that affects us all and that we almost never think about.
1. The first half of the book covers the dynamics of rivers. Everything from how they shape the landscape around them via erosion and sediment deposition, to how water reaches them (via overland flow, groundwater, etc), to how they get their shapes (meandering, braided, shallow, deep, etc), to how they are influenced by climate. The book opened my eyes to how many of the landscapes I have seen, especially in California, have been shaped by rivers. One thing that was especially interesting to me was how sediment is carried by rivers. Rivers have two qualities, "capacity" and "competence" - capacity is how much sediment load a river can carry, and competence is the maximum size of the individual pieces of sediment that they can carry. If a river's competence decreases, that will cause it to deposit sediment. Competence is controlled by variables such as river gradient and velocity. So, for example, as a river flows down a mountain valley, it will have high gradient/be fast and pick up a lot of gravel-sized sediment. When the river reaches the base of the mountain, it will slow down/spread out, reducing its competence and causing it to deposit the gravel-sized sediment. Therefore you will notice that at the bases of mountains, rivers will typically have large gravel bars. As the river flows towards the see, typical its gradient/velocity get smaller and smaller, causing finer and finer sediment to be deposited. The mouth of the river will have the finest sediments, basically clays and fine silts. Another interesting discussion in the book is about groundwater. Groundwater has always seemed mysterious to me but the book explained how it works pretty well. Under the land surface, there is typically an are called the "unsaturated zone" that is dry. But at a certain level called the water table, the earth becomes saturated with water. Groundwater moves very slowly, allowing the accumulation of gradients in the height of the water table. For example, it's possible that under a hill, the water table bulges up. These gradients in water table height cause groundwater to flow. For example, the high water table under the hill will slowly flow down and away from the hill. The height of a river during "base flow" (aka when there is no storm run-off or snow melt influencing flow) is typically at the height of the local water table. So for example there are rivers in California that flow all year despite the lack of summer rain because the groundwater in the neighboring mountains gives them a steady supply of water. However, in areas where river channels have been deepened/scoured (typically by human modification), the river might actually flow below the level of the water table, especially during droughts. This is really bad because it will cause groundwater to drain into the river, lowering the entire water table of the region. Alternatively, humans often pump groundwater out of the ground, causing the water table to lower. This can greatly reduce the flow of rivers as their water gets sucked into the ground to replenish the water table.
2. The impact of rivers on human life. The book discussed how rivers' shapes are determined by "bankfull stage", i.e. the water level at which a river reaches the top of its channel. This stage happened on average every 1-2 years, so it's basically the most frequent "significant" flow that the river experiences and therefore has the most influence in shaping the river. However, unmodified rivers experience discharge rates according to an approximately log-normal distribution apparently, with approximate recurrence rates such as the so-called "100-year flood". Although rare, discharge rates that cause rivers to overflow their banks are inevitable, and the flooded river will expand to its floodplain, where the much large "hydraulic radius" will cause the river to slow down and deposit sediment. Basically, you should think of the floodplain as being an integral part of the river, which it uses to store excess sediment. Almost any flat geographical area was or is a floodplain. And humans really shouldn't live on them without strong flood protections, as the devastation recently wrought by Hurricane Helene in North Carolina shows.
3. The impact of humans on rivers. The second half of the book goes into great detail about how human modifications affect rivers. Logging, for example, especially "tractor yarding", greatly increases the runoff and sediment load of rivers not because of the cleared trees but because of the compacted roads that are created in order to reach trees. Urbanization reduced sediment load but greatly shortens the "hydrograph response" of a river (how a storm will affect the flow of a river - peak flow will be much higher and much sooner) due to concrete/asphalt urban surfaces and how we modify our urban rivers by straightening them and lining their beds with concrete. The construction of levees often has perverse upstream and downstream effects - the narrowing of the river channel can cause the river to get "backed up" upstream of the levee, causing flooding. Meanwhile, during floods, the faster, higher flow of the river in the levee will also cause flooding downstream of the levee. This creates a race to levee the entire river, which can have dangerous consequences during especially large floods when the levee is at, near, or above capacity, which can cause levee failure. Humans also mine river channels, typically for gravel. The mining operations typically greatly alter the river channels both upstream and downstream of their operations as the river channel struggles to adjust to the huge depletion of sediment at the location of the operation. The biggest human impact of all, though, must be dams. Dams complete alter dammed rivers. First of all, dams capture basically all sediment, so rivers downstream of the dam are "sediment hungry" - they scour their beds for sediment, incising into the landscape (which can cause water table drainage) typically until the river becomes "armored" (its channel is full of huge rocks that the river is too weak to move). On the other hand, as tributaries enter the main channel, the reduced flow of the main channel means that finer-than-usual sediments are deposited in the channel, causing it to fill up and shrink. Both of these changes are devastating for salmon and related fish which like to lay their eggs in the gaps between pieces of gravel, which often either get swept away or filled with fine silt. By constricting the flow of the river, dams have converted braided rivers into meandering rivers.
4. The vast scale of water engineering in California. Before reading this book, I was so oblivious to the enormous extent to which California has engineered its water supplies. This was necessary because most of California's rain falls in the remote north, and most of the population land a lot of the agriculture happens in the south. We have dammed almost every major river in California. We have constructed massive aqueducts to transport that water, such as the Bay Area Aqueduct and the California Aqueduct. There are massive pumping stations located throughout the state that pump those waters over our mountains and into our population centers. Our flood control structures are also very interesting, especially the Yuba Bypass west of Sacramento, which is a huge area devoted to be an occasional floodplain for the Sacramento River which spares West Sacramento from being flooded. Lastly, the Sacramento-San Joaquin Delta is a hugely underappreciated part of the state. Much of our freshwater flows through the delta before being pumped into canals and aqueducts for human use. The delta is very vulnerable because it lies at sea level and is exposed to the salt waters of the San Francisco Bay, so increases in sea level will cause salt water to intrude into the delta.
There were many other interesting discussions in this book, from the specifics of California climate (such as how our cold-wet, cold-dry, warm-wet, and warm-dry storms form) to the ongoing impact of hydraulic mining in the 1800s, to the expected impacts of global warming/climate change. Overall, it was a really fascinating overview of an aspect of our environment that affects us all and that we almost never think about.
August 19, 2009
I'm still struggling through this one. He is a local author and a UC Davis professor, and the book is half a course on how rivers work and half about water policy and its failures, and I feel obliged to get through the first half in order to read the second half!
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