This is obvious to anyone who has a pond they take care of (probably not a lot of people), but most ponds are hybrid environments – part natural, and part man-made. This past couple of weeks have made this hybrid more conscious because of two events: 1) Algae bloom; and 2) Cleanout.
As a reminder, our pond is filled by water that flows down Two Mile Gulch which comes out of our southwest from a pipe about 200' higher than us. The flow is controlled by the local water district, and there's no "plan" for when they start and stop pumping. So man-made, but not predictable (believe me, I've asked).
This is a picture of the algae bloom that showed up a few days after they turned off the water. We've had our pond since 2006, and have never seen this kind of algae bloom… I'm guessing there was something in the pumped water, maybe as a result of fire retardant from the recent Cold Springs Fire. This website reports: "In still bodies of water like lakes and ponds, concentrated doses of retardant can immediately kill fish, or the nitrogen and phosphorus in retardants can lead to algae blooms that consumes oxygen and kills fish slowly over time." Thankfully we don't "keep" fish in our pond (because of the unpredictable nature of the water level).
The second aspect of the hybrid is itself a hybrid… the cleaning out of our pond a few times a year. It occurs because of the 150+ gallons/minute that the pump delivers (plus gravity) and what it picks up as it travels down the gulch naturally, ends up settling in our pond, which is the first flat area of slow-moving water.
Above is an example of the sediment forming two distinct layers, a lower layer of dense water-impermeable clay, and a top layer composed of sand and stones. Volume for volume, the former weighs about three times the latter. That dual layer pattern occurs throughout our pond, which is divided into an upper and lower part. The upper part where the water first enters is shallow (< 2' deep), then it flows into a deeper part (≈ 4') before it spills over our spillway/dam into the culvert and down the other side all the way to our Pine Brook Hills Dam about a half mile away as the crow flies. Depending on the time of year and the water flow, once or twice a year now the parts start to fill up and while it's running, we'll clean it out (see my earlier blog for this 'aha!') by first pulling the plug out of the bottom of the dam, and letting the water flow through the drain into the culvert, naturally taking sediment along with it. This week we spent a total of about 5 hours between the two of us "facilitating" this process, and moved about three cubic yards of material down the culvert. Moving the dense clay is harder because it's heavy, so it doesn't move well in the running water. But below is the end result that we're enjoying for a while longer until they turn the water off again and the pond drains.
