A planted dry streambed is the perfect alternative to lawn, but if you don’t understand the natural process behind rivers, yours will never look great. Thanks to technology, Google Earth allows everyone to study real river bed models from a bird’s-eye view. Aerial imagery in the dry Western states shows in real terms the dynamics of water flow and the way it influences how erosion creates the dry riverbed.
Look closely to see irregular, sweeping and almost artistic sand and gravel bars build or are swept away. This bears no resemblance to what some folks are calling a dry streambed, which is more like a linear pile of cobblestones than a natural feature.
When water moves quickly over land, its velocity is able to pick up sand and pebbles, carrying them along in suspension, which is the essence of soil erosion. As flowing water slows due to barriers, curves or irregular terrain, heavier pebbles drop out of suspension to create a gravel bar. When the water slows even more, often due to flatter topography, the smaller, lighter particles drop out, creating sand bars. This demonstrates why a streambed must be composed of different sized aggregates to recreate the surface diversity.
When the riverbed turns, there are other forces at play. Flowing water scours out banks on the outside of a curve, carrying away eroded soil and gravel. If the flow line then turns the opposite direction, it slows on the inside of the curve where sands fall out to make a beach. This is highly variable as water takes the path of least resistance, with the same process of scour and deposit occurring throughout the length of that river. With this knowledge, take another look at Google Earth to identify sand and gravel bars, cut banks and sandy beaches.
When looking at rounded or “river-run” gravel for your streambed, select a variety of aggregate sizes. Use coarse sand, pea gravel, larger river gravel and smaller boulders or cobbles, all of which share similar material coloring for a unified appearance. Start by setting your flow line with stakes and strings for alignment. Next, set your boulders at transition zones since they can influence flow. Then distribute the sand and gravelly materials along the sides in areas based on where water slows under those conditions.
In the arid West, wild plants tend to be more plentiful around boulders or groupings of large rocks along the edges of the streambed. This is because moisture is trapped beneath the huge stones long after the water ceases to flow. This tells why ornamental grasses and reeds belong adjacent to larger rock outcrops within the streambed composition.
In the wild, small plants pop up in the fine sandy beach parts of the bed. With deep fine rooting area unrestricted by rocks, fast-growing, deep-rooted plants survive best here.
Riparian trees and shrubs such as riverbank willows naturally occur here as nature’s way of holding ground with networks of fibrous roots. In the dry streambed, use your shrubby plants on the outside of curves where the cut bank would be. Further out from the gravel areas, place your trees, since they would be swept away by high water within a natural riverbed.
The streambed has long been used to create porous ground for succulents. This technique imports gravelly ground to elevate cold hardy alpines such as sempervivums and sedums where the soil is dense or poorly drained. It works equally well for tender succulents and desert plants in a residential garden where they might not thrive due to drainage issues.
Maureen Gilmer is an author, horticulturist and landscape designer. Learn more at www.MoPlants.com. Contact her at firstname.lastname@example.org or P.O. Box 891, Morongo Valley, CA 92256.