By Alicia Springer, UC Master Gardener of Butte County, November 2, 2018

When that first light rain fell over our region in early October, a collective sigh of relief fell as well. We all agree that rainwater is precious—so why do we try so hard to move it off our property? On city and suburban lots, we channel rainwater from roof gutter downspout to street gutter storm drain as directly as possible. We don't want rainwater pooling around our foundations or settling into muddy sumps in our gardens, so modern houses and driveways are engineered to clear the runoff. But why not capture that precious rainwater in the soil? “Slow it, spread it, sink it” is the rallying cry of this permaculture principle of rain capture, and with some fairly simple landscaping—small berms, swales, and basins that direct, detain, and filter water—we can invigorate our soil ecology, enhance our yard's aesthetic and habitat value, filter pollutants and help relieve over-tasked city storm drains.

Chicoans have a macro-example of this approach to storm water in our own Lindo Channel, which catches the overflow when Big Chico Creek runs high. While many towns direct excess storm water to concrete flood control culverts which whisk the water to treatment plants, Lindo Channel's runoff nourishes a riparian corridor of oaks, sycamores, willows, cottonwoods, and bay trees (along with nonnative brush). The stream flow recharges upper-level groundwater, and the greenery helps clear air pollution and offers sanctuary to birds and other animals large and small.

The goal of on-site rain capture is to allow water to percolate into the soil rather than to create holding ponds, which require more complex engineering (and can foster mosquito breeding). Any effective rain garden landscaping should discourage standing water and allow flow to infiltrate the soil within a day or two. This can be as simple as a gravel-lined trench (often called a french drain) leading from a downspout to a grassy meadow or planting bed. A more complicated retention basin might entail layers of soil, gravel, and sand designed for slow seepage. For the purposes of this brief introduction, let's focus on a versatile and relatively simple structure, the swale.

A swale is a shallow channel, bermed a bit on both sides, that lets gravity do the work of moving rainwater along a gently sloping course. The swale directs water away from structures and through a straight or meandering depression lined with river rock, grasses, or other vegetation that can tolerate periods of winter wet and summer dry (regional native plants generally fit the bill). A grassy, vegetative swale might be lined with cool-season native meadow grasses such as sedges (Carex species) or rushes (Juncus species) that thrive with winter moisture and go dormant over the dry summer. A rock-lined swale resembling a dry creek bed holds visual interest throughout the seasons. Water-loving plants can be sited closer to the flow, while more water-averse selections can keep their feet dry further up the banks.

Before planning your swale, observe your yard's topography and note where and how rainwater settles in the wet season. Are there existing declivities where the ground turns soggy? You might run a swale through that area to create better drainage, or you might construct an improved basin, but you should avoid adding water that will just sit there. Is there a natural slope you can work with? Great! Do you have areas of impacted clay that shed water like a duck's back? Cutting a gravel-lined swale through hardpan could be helpful, but difficult.

In general, your rainwater collection surface should slope downward from the water source, or you can create a minimum 2% slope. Steep slopes can create erosion, so avoid orienting your swale down slope in such areas.

Locate your swale:

• At least five feet away from structures without a basement, or ten feet from structures with a basement.
• Away from septic systems and leach fields
• Away from a tree's crown or major root zone to avoid root rot
• In full or partial sunlight to support plantings

Think of your roof as the watershed, the downspout or water chain as the headwaters, and a downpour as a flash flood. Construct your swale to withstand those rare, brief, but potentially powerful erosion-causing pressures. The channel should be at least 6 inches deep, and lined with grasses, gravel, or river rocks for the dry creek effect. Use the excavated soil to build up gently sloping berms on each side.

Anchor your artificial creek bed with larger stones, boulders, and deep-rooted plants such as deer grass (Muhenbergia rigens) and fescues (Festuca species). If space permits, larger shrubs can create a more dramatically varied look. Vegetation should be planted and at least partly established before the winter storms, so be prepared to irrigate in dry periods and through at least the first summer. Once well established, deep-rooted native plants will enjoy the cool season storm water and survive the summer heat.

The illustrations here depict a professionally-designed creek bed along the driveway of a family home in Chico near lower Bidwell Park. A DIY project could be smaller in length, width, and overall scale while still employing some of the elements that make this feature so delightful.

From a simple gravel channel to a dry creek to a more elaborate retention pond, the hardscape elements of a rain garden project may seem daunting without step-by-step instruction. “Coastal California Rain Gardens” published by University of California Agriculture and Natural Resources (UCANR), offers detailed information about planning, building, and maintaining a rain garden. Another useful guide is “Slow it, Spread it, Sink it! Rain Gardens and Swales” published by the Napa County Resource Conservation District. Both publications offer additional plant suggestions and resource listings.

Additional information and plant names used in photos:

A dry creek rain swale in a suburban garden landscape. The swale, roughly 50 feet long x 10 feet wide, runs along a sloping driveway. Redbud (Cercis occidentalis), deergrass (Muhlenbergia rigens), St. Catherine's Lace (a buckwheat, Eriogonum giganteum), Santa Barbara daisy (Erigeron karvinskianus) and California fuchsia (Epilobium canum) are among the native species seen.

Plants that tolerate winter moisture and summer drought line the banks of the swale. Idaho fescue (Festuca idahoensis) and California fuchsia (Epilobium canum) are seen. A redwood (Sequoia sempervirens) is on a raised bank so rainwater can irrigate but not saturate the root area. The redwood will require summer watering to stay healthy in the Butte County climate unless it's near a perennial stream.

A decorative rain chain at the ‘headwaters' of the creek bed directs flow from the roof gutter. Note that the creek bed is directed away from the base of the redwood, which is on a higher bank.

Boulders and deergrass (Muhlenbergia rigens) stabilize the slope and provide pleasing visual variety. Larger native plant selections screen the property. These include toyon (Heteromeles arbutifolia), mountain mahogany (Cercocarpus betuloides), deer grass, and redbud (Cercis occidentalis). 

Diagram of a grass-lined vegetative swale courtesy of Napa County Resource Conservation District.