Current
Dave Motes
Long Lake lies SW to NE in central Itasca county. Its gangly sequence of scoops and esker islands is familiar to students of Minnesota lakes. Long rocky points arc out into main basin, punctuating broad shallow bays. Right down the middle is an intermittent strip of skinny islands and reefs made of reddish stone ranging in size from pea to state-fair pumpkin. You can look up Long, but it will take a while; there are 8 by that name in the county and over a hundred in the state, if you count a couple dozen Little Longs and a Longyear and Long Meadow and Upper South Long (North Basin.)
Though our Long Lake isn’t very deep, it is pretty far north so swimming is only comfortable for grownups from about mid-July through early September. So it was probably August when my mother-in-law floated away and got me thinking about current. She was drifting in a blue plastic inflatable chair, the kind with arms and a back and a slot for your drink. The rest of us sat on the dock, watching the kids get filthy and feed the leeches, while Grandma read a book and spun idly in the wind thirty feet out.
That was when my thinking changed, the moment I realized that she was spinning idly in the wind, but not going anywhere. It was a typical Minnesota summer day: neatly spaced cumulus streamed before the usual steady SW wind at 15-20 knots. After supper the wind would lay down and the bass would eat poppers on weed edges, but right now the main lake was a highway of whitecap windchop. Our bay was covered from the fetch by a high wooded point, but fluky gusts still bounced the pontoon against the dock fender. Occasionally the breeze stole a page from grandma’s fingers and slapped it down again. Yet she stayed still.
I thought maybe she was riding at anchor, that somebody had rigged her a little anchor. I watched. There was no anchor, of course. She turned another page, and stayed roughly where she was. My daughter pegged her brother in the face with a handful of wet slime, so I had to parent and forgot about stationary mothers-in-law.
A while later, my wife said, in her worried-do-something-it’s-your-fault voice, my name.
My mother-in-law was no longer stationary. She was booking southwest toward the main lake at a considerable clip, unaware, absorbed in the latest literary outrage by Sidney Sheldon.
The weather had changed. The wind was down to a light breeze, earlier than usual. The waves were fading. The breeze was light. Things were calming down.
Some things. My mother-in-law was accelerating, steaming blithely along. My wife was accelerating, too. “Uh, honey?” she said, more urgently.
“Hang on,” I said. “I want to see what happens.”
Sure, it was a mistake, relationship-wise, but I got to see what happened: the chair, with the woman reading, cruised another forty or fifty yards upwind. There was only one explanation. The lake—a still, non-moving, highly stationary piece of water—had a sharp current, and it was setting against the wind, carrying my mother-in-law toward Hibbing.
At about that moment, once she was out of the wind shadow of the point, she arrived at a new equilibrium and began circling idly again.
“Glad it's a good book. If she looked up she’d totally panic,” I said. Just then, of course, she did look up, and totally panicked, and things got exciting for a few minutes.
But I didn’t forget to be curious about current.
Simple, right? the wind made current. But where? The southwest wind blew across the mouth of the bay, creating a circulating current within the bay and holding the floating object at the point where current and the wind balanced. When the wind faded, the current continued until the forces were equalized.
But when the wind blows a current in the northern hemisphere, the water flow shears off to the west due to the rotation of the earth: SwirlyToilet Force, I think it’s called. If Long Lake were 60 meters deep, the current would shear as much as 180°--that is, it would flow against the wind. Current isn’t just water flowing downhill. In fact, water flowing downhill is a drop in a very big current bucket.
Grandma’s blue plastic float might just as well have been a sled, because she was sliding down a hill. A small hill, but a hill nevertheless. The energy of the wind didn’t just move the water; it actually stacked it up on the windward side, and the current movement was simply the lake’s way to get itself back to level. Some local waterfowl hunters learned that the hard way last fall on a large shallow lake near here—a steady heavy wind blew all the water to the other side of the lake and left them sitting in unnavigable and unwadeable muck. The good news: they brought home dinner anyway, because passing ducks laughed themselves to death and fell in the bed of their pickup.
That stack-up is energy, of course. It can be solar—caused by heat imbalance over an entire globe—or can be wind energy, as we saw on Long. It can also be what it seems to be: simple gravity—as in the ‘fall’ of a river. It's all current.
In Minnesota we see evidence of a different kind of energy, and a different kind of current, in the form of glaciers. It’s trendy to call glaciers ‘rivers of ice,’ but another way to describe them would be as ‘currents of snow.’ The ice is mostly snow, compressed; the ice flows like water, mostly. We might assume, fueled by Discovery Channel videos, that glaciers are just rivers that go downhill and then splash into picturesque bays, very slowly. Mountain glaciers behave like mountain rivers, sure; all that slope makes for some fairly high-speed current, by glacial standards.
In mostly flat Minnesota, and in Long Lake, we can see how glacial currents flow. What they lack in speed, they make up for in energy.
Glaciers flow as much outward as downward. In fact, they make their own downward by stacking up as snowfall builds up faster than it can melt in summer, then is compressed into ice. Like wind energy stacking the water on the NE side of Long Lake, the energy is invested in coherent layers of ice. The weight of the ice presses outward (and downward--the earth’s crust actually compresses under glaciers.) The weight also causes a melted lubricating layer along the bottom of the glacial lobe, helping it to move along the surface of the earth, grinding rock into dust all the way. Eventually, the glacial lobe reaches its farthest extent, at which point another current takes over—the release of the energy stored as ice above the landscape, which becomes a kind of upstream current when that ice melts.
The current of stored ice-melt is usually gradually unleashed on the landscape, but glaciers make good dams. Minnesota had Glacial Lake Agassiz; when the water found an exit, the torrent carved the Minnesota River valley and began construction on the Mall of America. All that energy has to go someplace.
Oddly, the most visible effect of glacier current is the most recent, and perhaps the least muscular. We tend to think of glaciers as big lumbering landscape-crushers, like Shaquille O’Neil but with a better foul shot. But it is when the glacial currents fade and reverse that the main marks are made on the landscape. When the ice volume in a glacier stops increasing, the current of the glacier reverses. The load of stuff the glacier carries is dropped on the landscape and rearranged by the released water. When the wind fell on Long Lake, the current continued until the forces of gravity and wind were balanced. When each of the glaciations of the Quaternary Era ended, the system sought balance too.
Just like current in a lake or trout stream, the glacier behaves like a river. The violent, energetic flows of an expanding glacier pick up sediment—it’s still called sediment even if it’s a rock the size of a house. The sediment is moved as far as the current can take it, but then the current decreases and the sediment falls out in landforms like drumlins and eskers and Shaquilles. So it’s common in our landscape to find long scars or scrapes in the earth—the glacier's path—punctuated by stacks or winding hills of stone dropped as the glacier retreated and the meltwater organized the sediment. So Long Lake, where wind-created current follows the points and bays, was itself formed by the same current behavior. Ice shapes rock the way wind shapes water; it’s all current.
Harald Sverdrup was one of those early-20th century Scandinavian renaissance-men who mastered everything and made everybody else at Bergen East High School look bad. He studied physics, meteorology, and chemistry and even went exploring with Amundsen, where he satisfied the Norwegian craving for long sled-dog rides and short nights in scratchy sleeping bags. In the process he shifted his energies to oceanography, and eventually focused on current. At one point he became a Californian, if you can believe it. During WWII he analyzed waves, currents, weather, and tides. He was the one who advised Eisenhower that June 5 was best, June 6 OK too.
Sverdrup’s most enduring contribution is the unit that bears his name, perhaps the biggest unit on earth: the unit used to measure ocean currents. A sverdrup is water movement of one million cubic meters per second. The largest current on earth, the Antarctic Circumpolar Current, flows at a rate of 150 sverdrups. All of the rivers of the world combined move about 1 sverdrup. If the entire volume of Long Lake, crappies and leeches and Grandma and all, passed by a single point in a single second, it would be, roughly, 1/64 of a sverdrup.
Harald built on prior research to prove that currents aren’t just in it for the attention and the sexy unit all to themselves. He demonstrated that warm and cold currents redistributed heat across the globe, contributing to stable weather patterns and tippet-class world records and nifty names for sportfishing boats. Sverdrup didn’t create current, but I thank him every time I pass from the greenish inshore flounder-soup into the gut-pumping, mouth-watering cobalt clarity of the Gulf Stream. It’s not enough to say that the Stream is five or eight or twenty miles wide; we need Sverdrup to remind us that it’s that wide, and also yay deep and yay fast. Sverdrup’s neat numbers quantify the Stream: an enormity, a yang, a contained environment in which somewhere in one of those sverdrups swims a hungry white marlin dumbhungry enough to eat my fly.
Out there, of course, one would not expect to find my mother-in-law riding an inflatable blue plastic chair, reading a bodice-ripper. Current doesn’t connect things, at least not that obviously. Or quickly. But if she were out there we’d be reminded of something important, and not just that she probably would be a holding a nice dolphin or two. And pissed. Current, whether wind or gravity or temperature driven, is part of the big cycle that moves water, and ice and soil, and trout and therefore me and, really, everything, around the globe.
When I sit on the dock at Long Lake and contemplate Harald Sverdrup and glaciers, my mind runs down the lake to the outlet, which is a surprisingly small creek. That creek swamps its way toward Round, then through White Swan and Bluebill in a meandering maze of thick and thin watercourses that delights wildlife and real-estate agents. The water passes slowly through dozens of other lakes before delivering itself to the Prairie River. The Prairie lakes then rivers four or five times itself before meeting the Mississippi, which also does the fat-slow/thin-fast routine itself before it gets to Iowa and finally goes full river.
My mother-in-law would be annoyed if I dared to call her lake a “very slow moving river”—Minnesotans don’t build river cabins; nobody goes “up to da river.” But rivers are really high-current, low-depth lakes, and lakes are just slow-flow rivers full of lazy fish. This is especially obvious in Minnesota where glaciers were here just last week; glaciers of course are just tall, very slow rivers. Lakes are just dents in the landscape, and rivers are where the water overflows those dents. In a couple hundred thousand years (or time-sverdrups), the lakes will be mostly gone as the rivers carve themselves deeper beds and the lakes fill themselves with muck and tin cans and the bones of drifted-away in-laws. This is another unpopular geological fact among Minnesotans. By then maybe new glaciers will show up and clean up the mess.
Consider the commonwealth of my birth. In Virginia, every body of water is either a river, an estuary, a reservoir, or a swimming pool. There are two lakes often called “natural,” but, just like Minnesota lakes, they are actually just places where the current has slowed down for some reason. Mountain Lake is a valley full of water held by an earthquake-caused landslide, a current of stone; Lake Drummond is the lake in the center of the Dismal Swamp, a lake held back by a mucky festering biological dam of cypress roots, water moccasins, and Regent University. Today they’re stationary and still, but not for long.
Virginia may be lake-poor, but it’s river-rich; the lake-choked Mississippi may be the father of waters, but Virginia is crisscrossed with rivers, so many that the “discoverers” of the place ran out of names and took shortcuts to name the tributaries. Hence the Old Dominion equivalent of Who Built the Eiffel Tower: what are the three main tributaries of the Mattaponi? Why, the Matta, the Po, and the Ni, of course.
In current terms, there’s little difference between Minnesota and Virginia other than the current of time. The landscape there is old while Minnesota’s is young, or at least recently scraped. It’s easy to think of current as time while drifting in a raft on a river that passes through thousand-foot Short Hill Mountain. And by “through” I don’t mean, like, a tunnel. I mean the river ate the mountain. There’s a mountain, long and neat and straight; then there’s a big gap, then the mountain starts up again. In the gap there’s nothing left but current flowing over tasty ledges of the toughest rock, mountain-roots that hold way more smallmouth than any blue plastic inflatable chair. In Minnesota, the glaciers cut lakes; in Virginia, the rivers cut mountains, and current did both.
In fact east coast rivers have gorged on mountains long enough to take their current anywhere they please. The New River is not new—it’s old, among the world’s oldest. Because of its seniority, it gets to run backward, just like a glacier. It flows westward from Virginia—uphill—to become the Kanawha and join the Ohio then the Mississippi. Again: this river flows westward across the Appalachians. Geologists insist that means the river was there first. I scout the New’s class IVs very carefully before daring them in a framed whitewater raft, not the least embarrassed at the fact that those same rapids carried my mother’s great great great great grandfather unscouted to Ohio on a forty-foot wooden flatboat that weighed 12 tons.
Ahh, current. You can dam it or freeze it, but not for long, at least in glacial terms; current always wins. In Minnesota, the rivers are still on their appetizers, rolling a billion glacial boulders a few meters a year. When I wade the Mississippi or the St. Croix, cursing the bowling-ball cobble, a hundred feet below me the granite and basalt slumber, waiting for the next lobe to arrive and begin chewing away.