What Happens to Creeks During the Rain?
During the recent rain, you may have noticed rushing water in some of Santa Barbara’s creeks. Creeks are important features in any landscape because they channel water from rain and natural springs into larger bodies of water, including the ocean. Channelized or otherwise altered creeks, or those taken over by invasive plants, are affected differently by the extra volume of water than those that remain undisturbed or have been restored.
In an ideal, undisturbed watershed, where there is no development, manmade creek alterations, nor invasive plants, rainwater moves downhill towards the creek. Much of it seeps into the soil on its way, watering native plants and percolating downward to recharge the water table. This groundwater then feeds natural springs, which keep some creeks flowing year round. When water reaches a natural creek, it typically is further filtered by plants growing on the floodplain. This slow flow of the water towards the creek and through the soil helps purify it, removing contaminants such as bacteria from animal droppings, so that aquatic life in the creek and ocean can better thrive. When human activity, such as development, channelization, pollution, and the introduction of nonnative plants, is added to the watershed, this natural cycle is thrown out of balance.
Riparian ecosystems have evolved to filter out natural contaminants, but not human-caused pollution. This may include chemical pesticides, herbicides, and fertilizers used in gardens, soap from washing anything outside, human waste from homeless encampments, motor oil from roads, and other harmful substances, as well as litter. Anything potentially harmful that can be found on the ground will eventually be washed into the creeks, groundwater, and ocean when it rains. Keeping your garden chemical-free and watching where you dispose of anything unnatural will go a long way in protecting your watershed.
Concrete channels are put in creeks to stabilize the bank area and allow development there. Developing right on the bank of a natural creek would be a bad idea because of erosion, but channels have allowed buildings to be put much closer. In an average rainstorm, they keep the creek from widening with the extra volume of water as a natural creek might. However, channelization destroys riparian ecosystems, and this damage may extend further than the channel itself. Natural creeks usually follow a meandering path and contain rocks that slow down the water, whereas concrete allows rainwater to rush through a straight, frictionless channel at faster speeds. If the channelized creek turns into a natural one downstream, the “natural” area can be disturbed by the fast-moving water, and erosion may occur on the banks (which may prompt further manmade modifications). Pollution is also a greater problem in channelized creeks because the concrete does not help filter it out of the water the way a natural streambed might. Instead of channelizing creeks, communities should require larger creek setbacks (distances away from a creek that developers are allowed to build).
Creeks with Pipe-and-Wire Revetment
Pipe-and-wire revetment is another way that people have tried to stabilize creek banks in order to build closer to the creek. This feature resembles a wire fence placed against a steep bank and is meant to keep the soil from eroding into the creek. However, it has proven to not be very effective after a certain amount of time, as soil erodes from behind the wire during storms. In addition, if one side of the bank is stabilized with pipe-and-wire revetment, the force of the water becomes directed at the other side and requires it to be reinforced too. Eventually, we are left with a widened creekbed and defunct wire structures that do nothing but trap litter and impede wildlife access to the banks.
Watersheds with Abundant Hardscaping
Covering a piece of land with concrete or asphalt, such as to build a road or parking lot, affects the creek that the watershed drains into by causing runoff to move more quickly toward it. Like in a channelized creek, this water has less of a chance to be filtered of contaminants since there is not as much bare soil for it to percolate through. Landscaping features that encourage groundwater percolation, such as bioswales (artificial landscape elements resembling dry creekbeds and often containing native plants) may help reduce this impact.
Creeks with Dams and Other Barriers
Debris that washes into creeks during storms has been known to sometimes clog narrow parts of creeks and cause flooding. One way that people have tried to prevent this is by building debris basins. The basins are meant to capture items such as rocks and branches during a rainstorm while letting water flow past. Debris basins, like any manmade modification to a natural environment, have a few drawbacks in addition to their flood control benefits. The dams associated with debris basins act as barriers to steelhead trout, an endangered species, as the fish swim upstream. Cleaning out the basins, which is required to maintain their efficacy, disrupts the ecosystem. In addition, in a major storm event the dams can fail and release large quantities of water and debris. However, in many storms debris basins are often effective in preventing flooding downstream. Another manmade modification meant to stop flooding, slated to be installed in Montecito soon, is ring nets. These have not undergone much testing in the context of being placed across a creek to stop a debris flow, and it is far from guaranteed that they would actually stop one if it were to happen. It is quite possible that they could fill up with debris and burst, potentially worsening the problem, and they also will act as barriers to wildlife in the creek. Rapid, high-volume flow in the creeks after a storm is a natural process, and it is important that any manmade attempts to alter it are carefully tested, and their benefits and drawbacks analyzed, before being implemented.
Creeks Overwhelmed By Invasive Plants
Some of the debris that is known to cause flooding comes from nonnative plants that take over local creeks. One especially problematic species is arundo, a tall, bamboo-like plant native to the Middle East. Since being brought to areas such as Santa Barbara, it has rapidly taken over sections of creeks and is very difficult to eradicate. Arundo is very tall, and thus does not move easily downstream once uprooted by erosion. This can “back up” other debris and cause flooding.
Next time it rains, check out the water flow in your local creek. Monitoring an ecosystem such as a creek is key to understanding your role in protecting and interacting with it. For more information on Santa Barbara’s watersheds, see the website of the Santa Barbara Urban Creeks Council.