Wonder of the Week: what happened here?!

A mysteriously-transformed river.

I had no intention of writing three posts in a row featuring the Laramie River, but some amazing things have been going on there.  Three days ago it was cold enough for the first walk of the season on the frozen snow-covered surface (below).

Frozen river with thin blanket of snow.

That night it was very cold, -20º F (-29º C).  The next morning I intended to take a long walk downstream but when I got to the river I found ... no snow ... just smooth milky ice.  At the margins the ice was very thin, with water below.  How could this be?  The temperature had not been above 0º F for almost 30 hours.  Then another surprise -- about a quarter of a mile downstream a blanket of snow still covered the river, no more bare ice.  [You will just have to believe me.  It was too cold for photos, I had no camera.]

The next morning (with camera) I found the river had changed again, to milky green ice with large patches of hoar frost here and there.

Patch of hoar frost in distance.  Anchored stumps on right are habitat for fish.
Clear area on left is ice, not open water.

There were wonderful patterns and textures to photograph, especially the swirls.

What a mix -- lobes and swirls of clear ice, translucent ice, hoar frost-covered ice.

Lobe of smooth ice surrounded by ice with fine hoar frost.

A bit of symmetry.  This reflected forest is green, even in winter!

The next day the river was different again -- less green, maybe because the sun was out.

What a mystery!  Any ideas as to how the river surface was transformed?  It was too cold for the snow to melt -- maybe wind? (I don’t remember any), or sublimation?  And why was there still snow on the river downstream?

Lucky for me I called the right friend, who knew the answer -- flood!  Water moved over the frozen surface, melting snow, freezing to form milky ice.  The swirls were shallow flood fronts.  The first day, flooding had made it downstream only to where the snow blanket still covered the river (it's gone now).

These once were swirling lobes of shallow water flowing over ice, now frozen in place.

How the river “flooded” was another question.  Maybe an ice dam upstream broke.  Maybe freezing expanding ice forced water onto the surface.  What do you think?

Small shallow flood fronts along the river margin -- water forced out by freezing expanding ice?

I’ve written quite a few posts involving the Laramie River (aggregated here), probably just because it’s close to my house.  But it is a treasure!  It's a strip of riparian and wetland habitat that winds through an equally-interesting collection of past and present human industry -- the old WyoColo and modern-day Union Pacific railroads, a ranch supply warehouse, old packing sheds, a sawmill, the railroad tie plant that became a superfund site transformed into open space.  Interstate 80 is less than a quarter mile to the west and always audible.  But step off the paved path and most days you will discover something of interest.  Nature thrives even in the midst of Civilization.

Sparky suffers through yet another photo expedition along the Laramie River.