Streams and Trees: A Natural Pair

Streams are dynamic. A single storm can transform a peaceful backyard creek into a flowing torrent that erodes the stream, jumps its banks, runs across the lawn picking up and carrying down stream whatever is in its path. The once clear water can turn to a chocolate brown, muddy mixture of dirt, leaves, sticks, and perhaps the tennis ball or plastic toy from an upstream neighbor’s yard. Also present, but unseen, may be pet wastes and herbicides and fertilizers applied to a lawn or garden. This whole mixture of material, chemicals and trash moves downstream and eventually ends up in the lake.

Streams will always flood but impacts of these floods to both personal property, and the lake and stream systems can be reduced. Most importantly, we must look beyond one section of a stream to think of the whole stream. What happens at one point affects everything else down stream. And what happens on the land affects the stream.

The area next to a stream or lake has a great deal of influences. This area is known as the “riparian zone.” Riparian zones ideally are lush with deep-rooted vegetation, such as trees and shrubs. Good riparian trees include red maple, sycamores, cottonwoods, some birches, and green ash. Beneficial shrubs include dogwood, ninebark, viburnum and winter berry.

If you own streamside property, you can aid our streams by adding woody plants or managing a forested area already on the property. These strips of woody plants along stream sides are commonly referred to as buffers.

These plants along the shore take up nutrients, filter contaminants in runoff from storms, and stabilize the bank which helps reduce soil from washing into the stream. The trees and shrubs are good for fish and other creatures living in the water because they shade the stream keeping it cool and drop leaves and branches that become food and shelter. These benefits are reduced when grass is planted up to a stream’s edge and they are mostly lost when the areas along stream have bare, exposed bank.

When trees and shrubs are removed and replaced with roads, rooftops, driveways and other surfaces impervious to water, water cannot seep into the ground. Instead of slowly moving through the soil, water moves quickly over hard surfaces, arriving at the nearest stream much faster than it normally would. A graph of water levels in a creek over the course of the storm is called a hydrograph. A typical hydrograph in a watershed with lots of vegetation rises slowly and goes down slowly. The hydrograph rises higher and more quickly in a watershed with lots of pavement and other areas impervious to water.

“Flashy” streams are those that respond quickly to a rain event, rising rapidly with large differences between low flow and flood conditions. When more areas in a stream’s watershed are converted more pavements and other impervious surfaces, the stream becomes more flashy. This can lead to more extreme flow events that cause greater erosion. Because the water levels also drop down more quickly, because less waters is held back to feed the stream over time, low flows results causing poorer habitat for fish and invertebrates.

If we minimize the amount of water flowing directly from impervious surfaces into streams and let the water run through areas where there is at least some potential for water absorption, we can decrease the flashiness of a stream. Which brings us back to a band of trees and shrubs along the stream. Woody plants physically slow down water flowing over the surface of the ground and they create areas where there is much greater potential for water absorption. Those plants also take up the water in the warm seasons when they are actively growing and can help to mitigate the flashiness of a system.

These same plants by slowing water and allowing more flow through the soil can keep some pollutants from reaching the stream. For example, plants may intercept the nutrients washed off lawns, garden or agricultural fields. Plants can also take up some heavy metals and other chemicals. This filtering capacity means fewer contaminants reach the stream.

In addition to filtering runoff, woody vegetation helps to stabilize stream banks during floods. Tree and shrub roots act like huge fingers to hold back streamside soil that might otherwise be washed away. Vegetation that overhangs the stream, damping the impact of rain, cools the water, keeping it hospitable for trout. . When branches fall, the wood in streams slows water during storm events and can create good fish habitat. Additionally, insects, wildlife, and birds use vegetated stream banks for food, protection, and transportation corridors.

WHY NOT USE ROCKS?
For years, one of most common methods for stabilizing under-cut stream banks has been to use rocks either as riprap (large stones) or gabion baskets (wire mesh baskets filled with stone). While these slow bank erosion, they are less then ideal fixes. First, these methods are very costly and not very aesthetic. In addition, the water flowing past hard surfaces speeds up and often causes erosion downstream or on opposite banks. Lastly, stones do not have the same water filtering capacity or provide the wildlife habitat that vegetation provides. However, riprap, gabion baskets, and other methods using rock are sometimes the only option for a severely undercut bank or erosion that threatens the stability of nearby structures.

STRAIGHT CHANNELS PASS ALONG THE PROBLEM
Another historical solution to stream flooding was to straighten stream channels. While water may move quickly through a straightened area and reduce threats to adjacent buildings, the communities downstream will receive the floodwaters even more quickly. A stream’s natural meanders dissipate some of the energy. The water moving faster now has even more power to pick up larger rocks and woody debris and hurl them downstream at the next communities’ buildings and bridges. Channel straightening is especially a problem with small, headwater streams that convey water quickly during storms. If water moved slower in the small streams, they would reach the larger streams with less force, and have less potential for flooding.