Imagine seeing an eagle aloft on the spring’s warming thermals, with Stillwater below.

Downstream, near Hudson, Wis., the eagle sees the railroad swing bridge, and farther downstream still, the Interstate 94 bridge. Meanwhile, upstream, the big bird casts a sharp eye toward the railroad “High Bridge” a few miles above Stillwater, and to a series of torpedo-shaped islands that braid the river into channels of varying sizes, some navigable by powerboats, some not.

For generations, eagles, Canada geese, sandhill cranes, ducks and loons, among other birds, have returned to the St. Croix River Valley in spring.

Until last spring, never before had they seen such massive concrete and steel rise from the river. A new bridge.

Now imagine seeing a crappie, walleye, sauger, sturgeon or smallmouth bass swim beneath the frozen St. Croix.

These fish see the new bridge, too, from beneath the winter’s ice, where huge concrete blocks, or footings, rest on the river bottom, encasing the structure’s five double-columned support piers that rise to the river’s surface through more than 135 feet of bedrock, sandstone, gravel, mud and water.

These new footings will alter the St. Croix for fish and wildlife in ways unknown. Just as floods, droughts and other natural phenomenon have altered the river and the life it supports since time immemorial.

Now, as the St. Croix changes again, these most recent alterations, however great or slight, will beget more changes and more changes still. And because most things in nature are linked, fish and birds that call the St. Croix home, whether permanently or while in transit, will be connected by the new bridge in ways visible and invisible.

Example: Flowing water that washes over river rock and mud in front of, alongside and behind the new bridge footings will create new places for fish to feed or perhaps spawn, just as currents formed by the footings will carve new channels on the river bottom that some species — say sturgeon — might favor.

Similarly, if in late fall, river ice is slow to form around the bridge piers relative to the rest of the river, eagles may perch not far away, waiting for dead fish to pop up from beneath the ice just downstream of the new structures, making for easy pickings.

Savvy St. Croix anglers already are speculating about these possibilities and more, as they closely chronicle the new bridge’s construction toward its anticipated completion late next year.

It isn’t the expedient connection the structure will provide between Minnesota and Wisconsin that interests these river watchers. Nor do they necessarily think the new bridge will create more fish.

Instead, they believe the massive construction project will yield new, and perhaps more productive, fishing opportunities.

“There’s no doubt the new bridge will create fish habitat,” said Josh Stevenson, a fishing guide and owner of Blue Ribbon Bait shops in Oakdale and White Bear Lake.

A multispecies angler, Stevenson specializes in muskie fishing on the St. Croix.

“What’s unique about this bridge and its placement in the river is not only that it is being built and that changes are coming to the St. Croix that will affect fish,” Stevenson said. “But those changes are very close to the water just downstream, at the [Xcel Energy] King plant.

“Warm water the [power] plant discharges into the St. Croix already attracts a ton of fish. We’ll have to see how those fish might react to the new structure not far upstream. But it’s likely they will react in some way.”

Water depth where the new bridge crosses the St. Croix ranges between 20 and 25 feet. The bottom is mud, mud and more mud, extending about 85 feet below the river bottom. Beneath the mud are sand, gravel, sandstone and, finally, bedrock.

The new bridge’s support piers extend about 25 feet into the bedrock.

On the river bottom, meanwhile, encasing the piers, are the huge concrete blocks, or footings, measuring 43 feet by 43 feet. The footings extend about 10 feet up from the river bottom — or about half the distance to the surface.

“That’s a lot of new habitat,’’ said Dick “Griz” Grzywinski of St. Paul, a river guide whose home waters include the St. Croix and Mississippi. “Those big footings should draw crappies to them for sure, and probably other fish.’’

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The St. Croix River between the Arcola High Bridge a few miles upriver of Stillwater and the I-94 bridge at Hudson offers different types of fishing opportunities for different species of fish.

Just downstream of the High Bridge, for instance, smallmouth bass will take flies and surface baits tossed to the shallows near shore. Small spinners also are effective here. But jigs, not so much: The chance for a snag is too great, and the bottom is muddy.

Farther downstream, about half the distance between the High Bridge and the Stillwater Lift Bridge, small pockets of backwater hold largemouth bass. Also in this stretch, walleyes lurk off points and in channels between islands.

Move farther downstream, just downriver from the Lift Bridge, and crappies can be caught. Farther downstream still, on the Wisconsin side, sauger hang out on the lake portion of the river.

And jigging for walleyes can be productive a mile or so farther downstream still, either just above or just below the railroad swing bridge at Hudson.

If smallmouth bass are your target, pepper the downstream side of the rock jetty that extends to the swing bridge from the Wisconsin shore. And, in fall, trolling crankbaits with lead-core line in the channel between the swing bridge and the I-94 Bridge can put monster walleyes in your boat.

These various fish and fishing “hot spots” aren’t the result of magic, mystery or divine intervention.

Instead, bass, walleyes, crappies and other finned species can be found in these areas because that is where their preferred food, water temperature, water depth and/or currents — or lack thereof — are found.

Similarly, eagles and other birds find certain stretches of the river more to their liking — and appetites — than others.

How fish and wildlife will respond to the new bridge when it’s completed is anyone’s guess.

But more than a few river anglers are eager to find out.

“Those footings are already in the river,” said Bob Nasby, a fly fisherman who has fished the St. Croix for more than 50 years. “And while fishing won’t be allowed near them until the bridge is completed it’s likely they already are causing some fish to move to places they haven’t been before.

“Time will tell.”