I was fishing with a friend on the Mississippi River near Alma, Wisconsin, last week. We caught a limit of large yellow perch in the backwaters away from the traffic on the main channel. Even though the river is bounded by highways and railroad tracks on either side, it’s a wild area with beautiful views of the water, floodplain forest and bluffs.
Navigating on the river, positioning the boat and fishing amid the wind, waves and river current is thought-provoking.
We live in a world of moving fluids driven by gravity, solar radiation, and heat from within the earth. Fluids are substances that can flow, that don’t maintain a fixed shape. The air, water, even molten rock miles within the earth is fluid.
We think of ourselves as solid citizens but we are mostly fluid. Being land creatures, we carry our water around within us. About two-thirds of our body weight is water. Our blood, lymph fluid, even the cytoplasm within our cells flows. We draw in air with each breath to gain oxygen and expel water and carbon dioxide.
We think in familiar scales of time and space that relate to our size, attention span and life span. Our landscape appears to be unchanging except where people have made alterations. We expect to see the same mountains, hills, and valleys, rivers, lakes, ponds and ocean coasts. Under the action of wind, water, and ice, the land surface is sculpted.
Over a lifetime, we may barely notice that the surface of the ground in our neighborhood has been raised by sedimentation or lowered by erosion. We have seen some big changes to stream channels and floodplains in our area in recent years due to some large runoff events.
The Upper Mississippi River Valley was sculpted by floods. The St. Croix River once drained glacial Lake Duluth, a precursor to the present-day Lake Superior. The Minnesota River once drained the 11,000-square mile glacial Lake Agassiz.
Both rivers carried huge amounts of glacial meltwater and carved big valleys. The glaciers and meltwaters retreated, leaving the grand bluffs, river terraces and sand dunes along today’s Upper Mississippi and Minnesota Rivers. A good book describing these processes is “The Physical Geography of Wisconsin,” by Lawrence Martin, University of Wisconsin Press, published in 1965.
At large scales, air circulates in generally consistent patterns creating the characteristic climates in different parts of the world. The weather in our region is affected by the pattern of Pacific Ocean currents and temperatures and the globe-circling jet stream. At smaller scales, the wind, land and air temperatures, and interactions with water that produce our weather are really complicated. Advances in computers, satellites and monitoring equipment have allowed development of global atmospheric circulation models that perform well but predicting the probability, timing and amount of precipitation at any one place remains a challenge. Despite the vagaries of atmospheric dynamics, weather forecasting is getting pretty good.
Size matters when you’re in a moving fluid. We enjoy a breeze, lean into a wind, and take shelter from a tornado. An albatross with a 10-foot wingspan may fly for a whole year through the Southern Ocean without touching land where storms with winds 75 mph are common. Insects are small relative to the density of air, so a gnat flying through the air would be like us trying to walk through maple syrup.
Fluid density matters too. Water is about 800 times as heavy as air. Anyone who has dumped a canoe knows that water is heavy and that moving water carries a lot of power. Animals that live in rivers are adapted to living in this dense fluid. Fish in rivers are streamlined. They occupy places in the river where they don’t have to expend much energy by swimming to maintain their position and then rise up to grab food items as they drift by. Many species of insect larvae in rivers have flattened bodies and claws on their feet that enable them to hang onto rocks and vegetation, sheltered in the thin boundary layer of slower current.
Fluid dynamics is an engineering science for some and an inspiration for many. We have a real attraction to moving fluids, whether it is watching a campfire, a river, ocean surf, clouds, or birds soaring on the wind. Maybe that’s because we flow, too.