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And because streams' utility to us is as yet only partially understood, losing their inhabitants seems like a bad idea. "Life on Earth evolved to be interdependent, and we don't fully understand those relationships yet," Marx said. "We don't really know what we can do without. Each of the pieces has a different function -- soil absorbs groundwater, which people drink. Plants produce oxygen, so we can breathe. We eat the fish, which eat much smaller things. So you need the whole web. For me, being out on the creek gives a sense of completion."
Which is why the Golf Course Incident was so upsetting. "I mean, bottom line: It killed everything," she said. "We've never gotten any crayfish again. All we get these days are the larvae of flying insects, who fly in from other areas and deposit their eggs."
Once the group had gauged the state of Donaldson Run's vegetation, Marx dropped tiny tablets into clear vials of creek water to test levels of phosphate and dissolved oxygen, both markers of how much lawn fertilizer had washed into the stream since the previous census. Meanwhile, Radke crouched over the water and kneaded the undersides of softball-size stream cobbles to work loose some insect larvae (which would be returned to the stream later).
Spencer, Lane and another neighbor, Larry Finch, head of the neighborhood civic association, all of them in their seventies, lamented, not for the first time, that their knees and their eyes were not what they once were. "We could use some younger monitors," Lane observed. "Their eyes are so much keener."
But what they did have was institutional memory; the long view.
"The water's flowing fast this morning, really clean," Finch said.
"Well, we had three inches of rain this week," said Lane. "That's a gully washer. Turns the stream into a storm sewer. And every time we get a gully washer, we lose a few trees."
"We'd love to find a crayfish this morning," Spencer said wistfully.
"When we moved in, in 1966, we often would see crayfish along the stream," Finch said. He hasn't seen crayfish in those numbers for decades.
Radke's children splashed in the water downstream, near where an artificial reef of gray boulders had been installed three years ago to stem erosion. Instead, the reef, known as riprap and operating like a pinball flipper, simply redirected the water to the opposite bank, where it proceeded to gouge an even deeper divot out of the creek's flank.
"Ladies," Marx called out to Spencer and Lane, her head bent over the microscope. "Anybody got anything for me yet?" Having scraped the required number of samplings from under stream rocks, in stream pools and in the ruffled currents known as riffles -- the monitors now emptied the nets into a white plastic dishpan filled with two inches of creek water. They began poking around the pan with tweezers, lifting out tiny bits of leaf debris, in search of even smaller larvae. When they found a likely suspect, they sucked it up with an eyedropper and squirted it into a chamber of an empty plastic ice cube tray.
"See that bug?" Radke said to his daughter as they leaned over one of the trays. "They're swimming. Swim, swim, swim!"