The Seasonal Pond
This pattern is shaped by
Problem
When land is drained efficiently, water leaves quickly and predictably — but the creatures that depend on temporary water disappear with it. Toads, salamanders, fairy shrimp, and dozens of invertebrate species require pools that appear in spring and vanish by midsummer. Permanent ponds won't do: they harbor fish that eat the eggs. Drained land won't do: there's nowhere to breed. The tension is between the human desire for dry, usable ground and the ecological need for wet, temporary ground — a puddle that lasts exactly long enough.
Evidence and Discussion
Vernal pools — seasonal wetlands that fill with snowmelt and spring rain, then dry completely by late summer — are among the most productive habitats per square meter in temperate landscapes. The California Department of Fish and Wildlife documented over 200 species of plants and animals associated with vernal pools in the Central Valley, including 34 species found nowhere else. In eastern North America, the wood frog (*Lithobates sylvaticus*) and spotted salamander (*Amblyostoma maculatum*) breed exclusively in fishless temporary pools; their populations collapse when these pools are drained or made permanent. The breeding migration of spotted salamanders — hundreds of adults crossing roads on the first warm rain of spring — has become a conservation icon in New England, where towns like Amherst, Massachusetts close roads on "Big Night" to protect the crossing.
The hydrological function is equally specific. Seasonal ponds act as groundwater recharge points during the wet season, holding water long enough for slow infiltration rather than rapid runoff. A study of prairie potholes in the northern Great Plains by the U.S. Geological Survey found that these temporary wetlands store an average of 2.5 million acre-feet of floodwater annually across the region — water that would otherwise contribute to downstream flooding. Unlike permanent ponds, which lose water primarily to evaporation, seasonal ponds lose water primarily to infiltration and plant uptake, making them net contributors to the local water table.
Alexander's Pools and Streams (64) called for water in every neighborhood — "still water for swimming, and fast water for drinking" — but treated water as a permanent feature. The seasonal pond completes this vision by adding the temporal dimension: water that comes and goes with the year, marking the seasons as visibly as the trees. A pond that fills with snowmelt in March, hosts a chorus of frogs in April, shrinks through May, and becomes a wildflower meadow by July teaches the rhythm of the year in a way that a static pond never can. Children who grow up with a seasonal pond in their neighborhood know something true about water that children with only taps and drains do not.
Therefore
where the site has a natural low point or where grading can create one, build a shallow depression — no more than 450mm deep at center — that collects runoff from rain gardens, roofs, and adjacent landscape. Line the bottom with compacted clay or bentonite to slow drainage, but do not seal it completely; the pond should hold water for 8-12 weeks in spring but dry completely by late summer. Size the pond to hold at least 20 cubic meters of water at peak — roughly 6 meters in diameter at 400mm average depth. Plant the margins with native sedges and rushes; leave the center bare or with submerged vegetation only. The test: by the first week of August, the pond should be dry enough to walk across; by the second week of April, it should hold at least 200mm of standing water. If it dries too fast, add clay; if it holds water into summer, reduce the clay or increase the overflow connection to adjacent rain gardens.