Wild Side column: Brimming rivers
"Till last by Philip's farm I flow
To join the brimming river,
For men may come and men may go,
But I go on for ever."
from Alfred Lord Tennyson, "The Brook"
Spring is a great time to be out on or along rivers. Brimming with snowmelt and spring rain, rivers are up, sometimes out of their banks and into their floodplains. Fish are migrating to spawning places, young muskrats are searching out new territories, beavers are out scratching around in the middle of the day, songbirds are migrating through and wood ducks are nesting in hollow trees.
On the Kinnickinnic River, thousands of white suckers (Catostomous commersoni) spawn in fast riffle areas. Many of those fish migrate in from the St. Croix River. The spawning males have a red stripe on their side. Spawning occurs with one male on either side of a female. An individual female may spawn in a number of areas, often in different rapids. Raccoons, mink, herons, ospreys, eagles and even owls feast on the spawning white suckers. The fertilized eggs stick to the bottom and receive no care. Larval suckers hatch out in a couple weeks, depending on water temperature. Young suckers grow rapidly feeding mainly on algae that grows on the riverbed. Small suckers provide an abundant source of high-protein food for trout and fish-eating birds.
The Kinni had a moderate flood on March 29, reaching 390 cubic feet per second at the gage in Clifton Hollow. That's enough flow to fill up a two-car garage to the ceiling in about eight seconds. A couple days later an inch of snow and freezing nights disappointed those of us looking for spring and shut down runoff. The river flow subsided to a more normal 110 cubic feet per second, sustained by ground water base flow.
Rivers are conveyors of more than just water. When the Kinni is flooding, it carries more than 200 milligrams per liter of sediment suspended in the water. Even more coarse sediment (sand, gravel and rocks) slides along the riverbed. During the flood events, the river is a conveyor belt carrying more than 50 tons per day, or about three big dump trucks full, of fine suspended. Most of the Kinnickinnic River sediment finds its way to the St. Croix River where it has formed a delta into Lake St. Croix. During larger floods, sediment is also deposited in the Kinnickinnic River floodplain. The coarser sand drops out first along the riverbanks and finer silt gets deposited in floodplain areas where the water slows down. The hydropower dams in River Falls created impoundments that the river rapidly filled with sediment many years ago.
The St. Croix River was flowing at about 35,000 cubic feet per second on April 1 at the Prescott gage. That's a moderate spring flow for the St. Croix. The St. Croix River drains a mostly forested watershed and carries little sediment. The Mississippi River formed the natural levee (Point Douglas) that dams up Lake St. Croix at Prescott. Nearly all sediment carried by the St. Croix settles out in the lake, so very little gets discharged to the Mississippi River. You can see the brown sediment-laden Mississippi River (most of the sediment is from the Minnesota River) mix with the clearer St. Croix at the confluence.
Rivers also carry lots of organic matter. In the spring, dissolved and particulate matter from dead grasses and leaves gets washed into rivers. Bacteria and fungi grow on the plant particles, making small nutritious "sandwiches" for macroinvertebrates (small critters visible to the eye that don't have backbones; like aquatic insects, snails, worms and crayfish). Rivers carry plenty of larger woody debris. Trees and branches get hung up in the river channel, providing shelter for fish and a hard substrate for filter-feeding aquatic insects like caddisflies. Caddisfly larvae spin small nets like spider webs and filter out fine particulate organic matter to eat. Think of living in a strong wind with chunks of food flying by.
The high concentrations of dissolved organic matter in early spring runoff make for lots of brown foam on rivers. The "river meringue" often forms big billows below dams and waterfalls, and chunks blow off in the wind. Canoeists beware!
Aquatic plants and algae grow faster as daylight lengthens and the rivers warm up. In addition to the organic matter flowing in from the watershed, "home grown" plants like algae and pondweeds provide food for life in the rivers. In the Kinnickinnic River, the rocks scoured clean by floods turn fuzzy and brownish green. The brown fuzz is a periphyton community of algae, mostly one-celled glass-shelled diatoms (they are beautiful under a microscope). Many species of aquatic insects that are food for trout feed on the algae growing on the river bed. The cushiony beds of water buttercup begin to grow this time of year. These plants trap sand in their fine leaves and stems, creating mounds in the river channel. The plant mounds shelter lots of small crustaceans (scuds) that are also bread-and-butter food for trout. The water buttercup beds expand until they flower in late summer, restricting flow into channels between them.
Enjoy the spring when it gets here after this late winter. Look closer when out along a river — it's brimming with more than water.
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