Know Your Forest, South Fork Cascade River

There was a forest fire on the South Fork of the Cascade River in 2005 or 2006. I think it was 2005 so, from this point on, it will be referred to as the fire or burn of 2005. I was on a crew that had a contract with the U.S. Forest Service to survey the river that year and the fire kept us out. We did the survey the summer after the fire. 

The fire was allowed to burn because there were no assets like homes or other buildings at risk and fire is a natural process in the forest that has a lot of beneficial effects, despite the seeming destruction it causes. A good part of the burn occurred in the Glacier Peak Wilderness so this area provides an example of what a recent disturbance like a stand replacement fire looks like in a landscape unmodified by humans. The part of the burn that wasn’t in the wilderness was a mix of old growth timber and units that had been logged in the late 1950’s and early 1960’s. My dad planted a lot of the trees in those logging units after the logging was done.

The center section of the South Fork of the Cascade between a box canyon and the confluence with the Middle Fork has always been prone to landslides. This is probably due to the soil types and parent rock of the soils as well as steep slopes. There are a lot of avalanche tracks, and soil moisture is high much of this area. There are a lot of springs and a lot of wet ground in this area. All of these factors, and probably some that haven’t occurred to me, likely make this area very sensitive to disturbance. During the twenty or so years that I have been using this area, there were always slides in some state of activity in the center section of the valley. Most of the slides in finer soils follow the channels of small tributaries to the South Fork.

Immediately after the fire in 2005, a number of severe landslides occurred in the areas prone to sliding along the middle section of the South Fork. I have a hunch this was probably due to an increase in soil moisture.

A commonly held misconception is that if you remove trees, either by logging or fire, etc. stream flow decreases. While this can be true sometimes, it is not true in most situations for most of our forests. The respiration and growth of plants removes large amounts of water from the soil through a process call evapotranspiration. Living trees remove a significant amount of water from the soil. When trees die, the water remains in the soil. This is a well known phenomenon employed by some water districts to increase stream flow. It is explained well in the book Wildland Watershed Management by Donald R. Satterlund and Paul W. Adams 2^nd edition published in 1992. The dynamics of soil moisture are described in many areas of this book but Chapters 8 (pages 138 to 163) and 11 (pages 233 to 297) and pages 211-213 explain it best, with good summaries of concepts and ideas about soil moisture dynamics on pages 157 and 286.

In the case of the South Fork Cascade tributary slides, I think the soil moisture in these more sensitive areas increased after large swathes of timber were killed by the fire. This probably further destabilized already unstable soils and resulted in the large landslides. There was major flooding in 2006 which also probably resulted in more and bigger slides in these areas. I believe there were some large slide events prior to the 2006 floods but if the initial large slide events were triggered by the 2006 floods, I would note that the scale of the slides in 2006 was much greater than slides which resulted from major floods in 1996 and 2003.

Most of the large landslides went straight into the river and significantly increased the amount of sediment in the South Fork. This caused aggradation, a condition where the stream channel is basically choked with sediment. This caused the river to meander out of its normal channel which was now filled in and, in many areas, cut into the stream banks which caused further and erosion and sedimentation. Increased sedimentation and erosion can also destabilize areas upstream in a kind of chain reaction that causes erosion to run upstream.

Sedimentation is often blamed on human activity like logging and roads. This is certainly true, especially when too many roads are built in too small an area, or they are built on wet and unstable soils or on naturally unstable parts of landscape like valley headwalls or when roads aren’t maintained. But also pays to remember that sedimentation also occurs as part of natural processes that have very little or no direct human influence.

I once read a paper that explained how sedimentation is actually important in the natural course of things for providing spawning gravel for fish. In this paper, the author posited that, over time, spawning gravel gets gradually washed downstream and, at some point, it will become exhausted if it is not replenished, causing lack of spawning habitat.  The author also posited that periodic forest fires caused an increase in stream sedimentation which replenished spawning gravel.

Most of our plant and animal species are adapted to periodic disturbance and have quite a capacity to deal with disturbances. And many species actually depend on disturbance for their survival. The important thing to remember is that there is a balance. Too much sediment, and you have a shallow, braided river that warms up too easily and provides very little fish habitat. If the sediment is fine enough, it will actually smother salmonid eggs or prevent young fish from emerging from the gravel. Too little sediment and the fish don’t have enough gravel to spawn in and reproduce. In between these two extremes is a balance that our salmonid species need in order to reproduce and survive.

It appears that the South Fork Cascade River has now stabilized somewhat with fewer braided channels that flow less erratically and it is beginning to cut down through the sediments deposited after the fire and flush them downstream.

The forest that burned in the wilderness area was mostly old growth and outside the wilderness it was a mix of old growth and second growth. The new forest in the burn is in the slow process of growing back, which will take many years. The new forest will probably not look like the forest that just burned because it is being established under different conditions that are different than they were when the old forest was established, especially in the old growth areas where many trees were established hundreds of years ago.

Conditions such as climate, weather patterns and species present in adjacent intact forest stands that provide seed sources will influence what species of trees will become established in the burned area, how fast they will grow and how long they will live, among many other things. Climate and CO2 levels in the atmosphere will probably be major factors influencing forests that are established today.

One quite interesting example of a significant change in forests over long periods of time is the presence of western red cedar (Thuja plicata) in the Pacific Northwest. The archeological/paleontological record shows that when the glaciers retreated at the end of the last ice age, 10,0000 to 12,000 years ago, there was no red cedar here. If memory serves, this species didn’t appear in the Pacific Northwest until about 5000 to 8000 years ago, (I’m a little hazy on the exact time frame and, unfortunately, I don’t have the reference with me). At any rate, red cedar didn’t appear here until thousands of years after human occupation. This tree, because of its many uses, has been important to Pacific Northwest Native American cultures for thousands of years to the point that it seems inseparable from these cultures. But, for thousands of years, after the last ice age, it didn’t exist here. It was unknown to the earliest ancestors of the people of present day Native American cultures.  

The process of one forest being replaced by another, younger forest has undoubtedly been repeated here on the South Fork Cascade, as it has all over the Pacific Northwest, many times over, with or without human influence. Our forests are not static. They are always changing, sometimes rapidly as the result of fire, logging or other disturbances, sometimes at a pace too slow for us humans to notice. One thing is sure, no forest that exists today will be there, unchanged forever. At some point in time, it will be replaced by another forest. And I have a hunch that 5000 years from now Pacific Northwest forests may be quite different than the ones we know today.

A patch of old growth forest along the abandoned U.S. Forest Service road on the way to the South Fork Cascade River trailhead.

Second growth forest along the abandoned U.S. Forest Service road. Many units in this area were logged in the late 1950's and early 1960's. My dad helped replant this area when he worked for the U.S. Forest Service. In the foreground is a Douglas-fir (Pseudotsuga menziesii) stump about 5 feet in diameter. 

Second growth forest along abandoned U.S. Forest Service road, logged and replanted in the late 1950's or early 1960's. 

Second growth forest logged and replanted in the late 1950's or early 1960's that burned in the fire of 2005. This area is outside the Glacier Peak Wilderness. Most of the second growth trees here survived. In other spots, they were killed by the fire. 

Second growth forest logged and replanted in the late 1950's or early 1960's that burned in the fire of 2005. This area is outside the Glacier Peak Wilderness. This photo shows a number of stumps and logs left after this area was logged. Again, the second growth trees survived in this area. 

Second growth forest logged and replanted in the late 1950's or early 1960's that burned in the fire of 2005. This area is outside the Glacier Peak Wilderness. This photo is meant to be an example of what the forest floor looked like after the fire. It was largely bare and black for a number of years after the burn. This photo is fairly representative except, for the first few years after the fire, there were no sticks on the ground because they had all burned. The sticks in this photo are dead limbs that have fallen off the trees and have accumulated over time. It also took several years for much of the understory vegetation to grow back. 

Landslide that occurred a year or two after the fire. This slide crossed the abandoned U.S. Forest Service road and took part of it out. This slide has now grown back so thickly with shrubs that it is nearly impossible to see. The logs in the photo were carried down the hill by the slide. 

Burned old growth forest above the abandoned U.S. Forest Service road. One can see that not every tree was killed but many were. Note the thick growth of shrubs and understory vegetation. 

Burned old growth forest above the abandoned U.S. Forest Service road. Again, one can see that many, but not all, of the trees were killed. 

Burned old growth forest below the abandoned U.S. Forest Service road.

Fallen old growth Douglas-fir along the abandoned U.S. Forest Service road. This tree may have survived the fire and fell at a later date due to changes in the soil caused by the fire or by causes unrelated to the fire, something simpler, like old age. 

Wild blackberries or dewberries (Rubus ursinus). These berries were growing just to my right in the previous photo.  These are a species that benefits from disturbances that open up the forest. In my early post in July about wild blackberries, I go into greater detail about how these berries react to such disturbances. These berries provide food for many species of birds and animals. 

Large landslide that took out the abandoned U.S. Forest Service road. This slide looks like it is still active to some degree or was recently active because there is a lot of exposed mineral soil and very little vegetation on the sides of the gully. The bare rock in the middle of the channel indicates that there is running water here for at least part of the year which washes the fine soil off the larger rock. Sometimes road beds can trigger landslides by trapping water which saturates and destabilizes the road bed to the point where it collapses. Plugged culverts are common culprits in these type events. As the hill begins to slide, sometimes a chain reaction can occur. Erosion and sliding moves up a channel or valley as collapsing material removes support from the material above it which collapses in turn, further removing support and so on. This may have happened here or the source of this slide may have come from above the road. In any event, the burn likely exacerbated the situation by increasing overall soil moisture on this slope. 

Looking down the hill at the slide from the previous photo. In this case the material that slid off the hillside didn't appear to reach the river channel. 

Landslide in the Glacier Peak Wilderness in the burn area. The landslide has come down into the river channel and partially blocked it, causing large amounts of sediment to be built up above it and the river to cut into the bank at the right side of the photo. This caused that bank to slump into the river. This is in the wilderness area so it has never been directly modified by humans. The restriction caused by the landslide and subsequent sediment storage at this point in the stream is exacerbated by the fact that there is another stream restriction here caused by a narrowing of the valley so this spot was already prone to excessive sediment buildup and cutting. Sediment storage is generally a good thing but large buildups like this can also cause problems like stream meandering and cutting. 

Burn in old growth forest along the trail in the Glacier Peak Wilderness. This stand of timber was dominated by western hemlock (Tsuga heterophylla) and Pacific silver fir (Abies amabilis). Again this area has not been directly modified by humans. Also note that not every tree was killed by the fire. These surviving trees will provide seed sources to get the next forest established. This is also an opportunity for species like Douglas-fir and red alder (Alnus rubra) which are not shade tolerant and whose seedlings need large openings to get established. 

A section of trail through the burn. The trail is mostly hidden by the thick growth of shrubs and forbs. Many of these understory plants such as red elderberry (Sambucus racemosa) and huckleberries/blueberries (Vaccinium species) will provide important food sources for many species of birds and animals. 

A section of trail through the burn. The snags and logs from the trees killed by the burn will provide important "lifeboat" habitat for many species so they can survive into the next forest. Fungi and insects will eat and break down the dead wood. Woodpeckers will eat many of the insects in the snags and logs. The downed logs will provide cover for amphibians and other small animals. The logs will also provide a substrate for species such as blueberries or huckleberries to grow on. 

Slide in burn area in Glacier Peak Wilderness. This photo is looking upslope at the slide. Water obviously runs down this channel for part of the year as evidenced by the bare, water washed rock. 

 This photo is looking upstream at the South Fork Cascade River from the slide in the previous photo. The large bare gravel bar seen through the trees used to be the river channel. Note the large number of red alder seedlings that have established at the edges of the slide. These are growing on an earlier, larger slide. Red alder grows very fast, though the growing season here is shorter than at lower elevations. These alders are probably 3 to 4 years old. 

Looking downstream at slide pictured in previous two photos. Note the cut banks on the far side of the river and the trees fallen in from them. This slide and a large log jam just upstream have diverted the river so it is cutting a new channel through the forest. Major flooding in 2006 probably also encouraged slides in this area. 

Looking downstream at the slide pictured in the previous three photos. This photo was taken from the old river channel. The slide has blocked the river channel, diverting most of the river flow to the left (not visible in this photo) where it cut a new channel through the forest. There is still some flow here but the bulk of the river has been diverted. It is not obvious from this photo but there is a large log jam near the center of the frame. This jam probably also played a big role in diverting the river. 

Looking upstream from large log jam at former river channel. The main flow of the river has been diverted to the right of this photo and into the forest (left in previous photo). This photo is rather crooked for which I apologize. 

Looking downslope at another slide area in the center section of the South Fork Cascade in the burn area in the Glacier Peak Wilderness. The river channel is just out of sight over the break of the hill. The large cut bank is on the other side of the river and is about 30 feet high. The slide pushes the river into the bank, resulting in the cutting. The fine brown debris on everything has been carried to this point by an avalanche and deposited when the snow melted away. 

Looking upslope at the slide pictured in the previous photo. The burn is not evident in this photo but I looked at air photos on Google Earth and verified that it did, indeed extend into this area and likely had an effect on this slide. 

Looking upslope in the valley of the slide area pictured in the previous two photos. This photo illustrates that this area is prone to slides. Note the distinct line between the smaller and larger trees near the center of the frame. The smaller trees occupy an area where an avalanche came through and wiped out all of the trees that were growing there before. Even before the fire there were probably large amounts of sediment and debris being delivered to the river channel on a regular basis. 

Looking downslope at large slide near the top of the center section of the South Fork Cascade. This slide is not too far below a narrow box canyon that the river flows through. Note the large cut bank to the left of center frame. This slide has been somewhat active over the 20 years that I have known this place but it really blew out after the fire. 

Looking upstream at the slide pictured in the previous frame. The burn is not visible from here. It burned across the head of this valley much higher on the hill.

Looking downstream at the large slide pictured in the previous two photos.  This area is just beginning to revegetate. The more recently active parts of the slide are still bare. Note the large areas of bare mineral soil. Note the large deposits of gravel in the stream bed and the large cut banks downstream. The massive input of sediment from this slide filled the river channel causing it to run all over the valley in shallow multiple or braided channels on top of the deposit. This type of stream with shallow braided channels over bare gravel and boulders is poor fish habitat. The river has now cut down through through much of the sediment deposit, creating a single channel with deeper water that should provide better fish habitat. When I first came to this spot a year or two after the burn and the initial slide, it looked like a moonscape. The river channel was almost completely barren with little or no vegetation. This area is in the Glacier Peak Wilderness and has had no direct human modification, no logging and no roads. So this massive sediment input is the result of purely natural causes. Conditions that encouraged large landslides in this section of the river were also probably exacerbated during major flooding in 2006.

Downstream edge of the large landslide shown in previous photo looking downslope. This illustrates what I had written about some of the areas in the center section of the South Fork being sensitive and being in a state of activity during most years. This is an older section of the slide that is more stable and is beginning to revegetate. This area has looked about like this for the twenty years that I have walked over it, though I think overall there was more vegetation before the burn and big slide. 

Looking upslope at area in previous photo. 

Looking upstream at the South Fork Cascade River just above the confluence with the Middle Fork. In 2006, when we did the survey here, the gravel bars pictured here were mostly bare and the stream was often in shallow braided channels with some of the flow probably subsurface in the sediment deposit. The river has now cut down through the sediment deposit several feet and is a deeper, single channel. After the initial slides, the river surface was about where the thick growth of alders on the far side of the stream is now. The cut bank on the far side of the stream is about 4 feet high. 

Looking downstream at the South Fork Cascade River in about the same spot as the previous photo. Again, in 2006, when we did the stream survey, most of this area was bare rock except for the larger trees sticking up above the lower carpet of alder. 

Same spot as previous two photos showing closer view of sediment deposit. 

Wild ginger (Asarum caudatum). This photo was taken on the abandoned U.S. Forest Service road up the South Fork of the Cascade. Ginger seems to have an affinity for old roads, often growing on them in large patches. This is rather curious since road beds are generally composed of compacted gravel and fill rock which is pretty hostile to plants. However, ginger also has an affinity for wet ground and springs in other areas which might be an indicator that the road fill here has a high moisture content, which would not be a good thing. However, ginger grows in drier areas as well and this area doesn't look particularly wet.

Different view of the patch of ginger in previous photo.