Taylor Creek Conditions
Science of the Canyon
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Geologic history and composition
Thousands of years ago, the Seattle area was covered by glaciers that slowly carved out the hills and valleys that make up the city’s present-day landscape. The glaciers were so deep and heavy that they created a dense layer of soil and sediment that Taylor Creek is currently eroding. Although the layer is compact and hard, it is much more erodible than bedrock, and can erode fairly rapidly.
The streambed of Taylor Creek is mostly a thin layer of gravel and sand sitting on top of this compacted glacial material. During storms and heavy rainfall, the water in Taylor Creek is deep and fast and can scour through this gravel and sand layer, then erode into the glacial deposits. This is happening much faster in areas where the slope of the stream channel is steep.
Causes of erosion
Taylor Creek and Dead Horse Canyon have been fundamentally altered by human intervention, which has exacerbated natural erosion processes. With the construction of the Lake Washington Ship Canal and the Ballard Locks in 1916, Lake Washington’s water level dropped by nearly 9 feet. Other factors such as urbanization and the creation of impervious surfaces, such as roadways and buildings, has led to more surface water being directed to the Taylor Creek Watershed.
Taylor's Mill, 1910 (photo courtesy of Rainier Valley Historical Society)
Logging of the canyon at the turn of the century removed trees that would have fallen into the creek channel and naturally created barriers that keep sediment in the canyon. Trees also slow the flow of stormwater into the stream channel and allow it to instead soak into the forest soil, further reducing erosion.
It was also common logging practice in the 19th and early 20th century to float logs down streams for processing at mills. It is likely that logging companies employed this practice when getting lumber to the Taylor Mill at the mouth of the channel, damaging the streambed, destabilizing the streambanks, and removing logjams that naturally slowed the flow of the water. The stream is still slowly adjusting to these erosive effects.
Process and stage of erosion
When a stream has eroded itself downward into a narrow, gully-like shape, it is described as being "incised." An incised stream can no longer flow out onto its floodplain because, during storms, the water is all contained within the deepened, eroded channel. Incised streams tend to continue eroding, first downward, then along their sides, as the banks become taller and less stable.
The force of the water is so strong that gravel and sand cannot accumulate, except in a thin layer at the bottom. This erosion is compounded by the fact that there are hardly any logs to slow down the flow of water.
Taylor Creek is stuck in this intermediate state, where it cannot stabilize or accumulate any gravel and sand in the channel.
Instead, the channel quickly erodes and transports sediment in the lower stream channel near the lake and on the lakeshore itself, forming a wide, shallow area called a delta. A delta has always existed at Taylor Creek, but its growth has accelerated during the last century. Later in the Open House, we’ll discuss some of the impacts of the enlarged delta on habitat.
Increased risk of landslides
The advanced channel incision and erosion also increases the risk of landslides. The slopes of Dead Horse Canyon are so steep that the loose surface topsoils slide down the dense glacial material, and landslides continue to occur. The problem is that when these landslides occur, there is no way for them to accumulate in the bottom of the canyon, as the fast-moving water in the incised stream channel quickly carries the debris downstream. Slowing the water flow in Taylor Creek would give sediment a chance to settle and raise the level of the streambed, slowing erosion.
SPU is helping develop a restoration plan to address the large amount of sediment generation within the ravine. In 2020, our team identified that landslides are by far the largest source of sediment within Dead Horse Canyon, accounting for 70% to 87% of the annual sediment load. We also studied soil layering to help us estimate where landslides are most likely to occur. The field investigation identified gullying (erosion from surface flow down the ravine walls) and active channel bed and bank erosion as the other primary sediment sources.