The forest you currently see on your property is the way it is because of a combination of factors that shaped it as it was growing. The most important forest-shaping factors are soil, climate, and human actions.
Water, in the form of rain and snow, or
precipitation, is a vital component of the forest. However, it’s
a factor about which there is little a landowner can do. Unlike in
agriculture, irrigation is not feasible during drought years on most
forest lands.
Nevertheless, land owners do play a pivotal role in affecting how
rainfall reaches the streams running across their lands and how much
sediment, or loose soil, these streams are carrying. Poorly managed
forest streams can cause accelerated erosion from your property which
reduces its productive capacity and degrades fish and wildlife
habitat.
In order to understand how to safeguard your property from accelerated erosion, it is first necessary to understand how streams work.
Rain and snow falling on forested lands is in the middle of a cycle between the land, sea, and atmosphere, known as the hydrologic cycle. When it reaches the forest floor, this water either seeps directly into the soil (this process is called infiltration), or runs off. The water flowing off the land is referred to as runoff. Runoff concentrates into streams as it flows through the forest.
The main job of streams is to move water and sediment over long distances. Over a geologic time scale of thousands of years, water running across the land wears down mountains and transports and deposits sediments elsewhere. Erosion and sedimentation forms plains, plateaus, valleys, river flats, and deltas. Over the shorter term, such as a human lifespan, most landowners would prefer to minimize erosion as much as possible, or at least not contribute to it so that their landscape remains relatively stable.
As water moves sediment through a landscape, it forms channels. The function of the channel is to collect and funnel water for transportation downstream. The channel also stores sediment.
The shape of a channel is determined by the amount of water and sediment it is carrying. As these amounts change, so does the channel’s shape.
Many stream channels have meanders or curves. Meanders serve to slow down the stream’s water or stream flow.
The width and depth of a stream channel determine its capacity to carry water. If the amount of stream flow exceeds the capacity of a stream channel to handle it, then water may spill out of the channel onto adjacent lands. The land adjacent to streams that is periodically flooded is called the floodplain. On average, a natural stream will overtop its banks and flood about once every two years.
Flooding is a natural and necessary part of stream systems. When water overflows its channel the flood waters carry sediments to the floodplain maintaining its level and adding vital nutrients to the soil.
Flooding is also vital to fish and wildlife which have evolved with it and are adapted to it. For example, annual floods trigger the migration of salmon and steelhead up coastal streams. Some plants that live next to streams depend on floods to spread their seed or reproduce from sprouting. Floods flush out sediment that otherwise would accumulate in streams.
Runoff flows through the forest in predictable directions. Of course, the force of gravity determines that water flows downhill! In hilly or mountainous terrain, it is not difficult to figure out which way is downhill. However, it does take some experience and a good topographical map to understand how all the downhills connect, and into which streams water from a particular piece of land will drain.
Using a topographical map, it’s possible to trace out the units of land which are connected by the fact that the water flowing off them collects to a common stream. This unit of land is called a watershed. A watershed is a unit of land that collects, concentrates, and conveys water. The boundaries of a watershed are determined by the shape of the land. A watershed’s boundary is a line that connects ridges or high points that determine which direction runoff flows (Figure 1).
Watershed boundaries can be drawn at small and large scales, depending on the size of the drainage network that you wish to understand. Smaller watersheds are nested within larger ones. The smallest are called "first-order" watersheds because they cannot be subdivided any further. First-order watersheds join to create second-order watersheds. Springs and seasonal creeks have first- or second-order watersheds. When second-order watersheds join, they create a third-order watershed, and so on. Large rivers and lakes usually have fifth-order or larger watersheds.
The amount of time it takes for runoff to reach a stream depends on a number of factors. In general, land managers prefer that runoff from precipitation reach the stream as slowly as possible. This is because the quicker storm runoff reaches the stream, the more water will be in the stream at once and, thus, the more erosive power it will have to carry away sediment.
Figure 2 is called a hydrograph and illustrates this idea. A hydrograph shows the amount of rain falling on a watershed over time. In Figure 2, part (a), we see a hydrograph of a rain storm.
This hydrograph can be compared to Figure 2, part (b), which shows the stream flow which drains the same watershed. Note that it takes some time for any increase in stream flow to occur. This lag time occurs because it takes time for water from the farthest point in the watershed to reach the stream. This lag time is called the watershed’s response time.
Figure 2, parts (a) and (b), show that a watershed really is a unit of land which stores water. As we all know, many streams continue to flow even when it’s not raining. This is because only a part of the stream’s flow is a direct response to runoff. The remainder of the flow, or base flow, comes from water which has infiltrated the soil and flows to the stream underground. Underground flow is generally slower than runoff so that a watershed with a high infiltration and low runoff rate will have a long response time.
In addition to the infiltration rate of the
watershed, all watersheds have inherent characteristics which affect
their response times. The amount of time it takes for rainfall to
reach a stream depends on:
- watershed size (order)
- slope
- number and arrangement of streams
- human management
In general, the smaller the watershed, the faster precipitation collects in streams, since it has less distance to travel. Watersheds with greater slope respond more quickly than flatter ones. The more channels that exist in a watershed, the less distance runoff must travel to reach them and so the quicker the response time.
Human management actions can change a watershed’s response time. Construction of building, roads, and parking lots reduces the amount of rainfall that infiltrates the ground and increases the amount that runs off. This will decrease the amount of time it takes for rainfall to reach streams. This means that a greater volume of water is in the stream at any one time and, therefore, it has more power to erode stream banks and carry away sediment.
Certain aspects of timber harvesting and
development cause changes in a watershed’s response time.
Response time is shortened when the infiltration rate of rainfall
into soil is decreased and when underground flow is intercepted and
converted to runoff.
These changes are caused by:
- soil compaction
- removal of litter layers
- road construction
Some timber harvesting methods require heavy machinery to drive over a lot of ground, which compacts soil and reduces infiltration of rain and so increases runoff. Removal of the litter layer also leads to reduced infiltration and increased runoff. Road construction introduces impervious surfaces which falling rain cannot infiltrate, thereby, increasing runoff. Roads intercept underground water flow bringing it to the surface and speeding its delivery to a stream.
Keeping erosion under control requires management of how your watershed handles precipitation during timber harvest and road and home construction. The basic goal is to ensure that precipitation continues to reach streams as slowly as possible in order to deprive it of the opportunity to pick up speed and erosive power.
Good watershed management includes the following elements:
- good road building and maintenance
- refraining from changing stream channels
- good timber harvest techniques
Proper location of roads is important for maintaining watersheds. Generally, roads built too close to streams supply sediment and funnel water into them. Adequate maintenance is required so that eroding sections of road are fixed and the supply of sediment to streams is minimized. Properly draining water off of roads will protect their surfaces from erosion.
Many landowners feel that they have no choice but to build levees to confine their streams to protect homes or other buildings from flooding. Unfortunately, levees, in addition to often being ineffective, result in stream water traveling faster within the channel, thereby, increasing the stream’s erosive power. The wisest course of action for landowners who contemplate building a structure is to build it away from the stream’s floodplain, thereby, eliminating the need to build levees altogether.
Channel straightening can also cause problems for landowners because removal of the stream’s meanders shortens the distance that water must flow to reach a point and, therefore, speeds it up, increasing its erosive power. Avoiding changing the channel’s shape will avoid some of these problems.
A good timber management practice is to leave zones of forest unharvested and untouched next to streams. These strips, known as buffer strips, function to slow down runoff and filter out sediment when runoff does reach the stream. Leaving vegetation next to the stream also strengthens the banks and helps it to withstand the erosive power of the stream.
Other techniques protect the soil’s infiltration capacity by minimizing soil compaction and retaining a litter layer so that the amount of runoff is not increased.
BASE FLOW: That portion of
stream flow which comes from precipitation that has infiltrated the
soil and flowed to the stream underground. This is in contrast to the
stream flow which comes from runoff during rain storms.
BUFFER STRIPS: Zones of vegetation left
unharvested next to streams.
FLOODPLAIN: The land adjacent to streams
that is periodically flooded.
MEANDERS: The bends in a stream channel that
serve to slow down stream flow, by forcing the water to cover more
distance to reach a point than if it were traveling in a straight
line.
STREAM FLOW: The amount of water flowing in
a stream. This is often measured in units of cubic feet of water
flowing past a cross section of stream per second.
SEDIMENT: Soil particles which are carried by
a stream.
WATERSHED: A unit of land that collects,
concentrates, and conveys water.
WATERSHED RESPONSE TIME: The
amount of time it takes for precipitation falling on a watershed to
reach the stream.
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Publication of this series was in part funded by the
California Department of Forestry and Fire Protection
under Contract numbers 8CA96027 and 8CA96028