As I mentioned last week, recent ice jams on the Platte River caused some flooding in some of our prairies. I was able to photograph the flooding from a couple perspectives. On February 5, we got some aerial footage with our drone, and then late last week, I walked through some of the flooded area after most of the water had receded.
This road was closed due to flooding. Everything to the right (north) of the road is prairie.
Another shot of the same area from higher up. The river can be seen at the very left edge of the photo. A windmill can be seen in the prairie near the right edge.
This was and will be an ecologically-interesting event from several standpoints. Floating ice and ice jams have been an important component of the Platte River ecosystem for a very long time. Ice likely played a critical historic role by helping to scour vegetation from sandbars, allowing that sediment to be carried off and deposited elsewhere, and creating valuable habitat for many species – including the ducks, geese, cranes, and other water birds that pass through during migration. Today, we see much less of that scouring and many sandbar islands and banks have become relatively permanent and covered by trees and other perennial vegetation.
River water flowing through prairie. Most Platte River floodplain prairies still have their river-formed topography of sloughs (old river channels) and ridges (old sandbars). During the flood, many of those sloughs became active (albeit temporary) river channels again.
Before the Platte River’s channels were stabilized and restricted to their current locations by human activity, ice jam flooding might have been an important driver of the shifting of channel locations over time as well. This year’s flood created new river channels through floodplain prairies and woodland, but as the water receded, it returned to its stabilized channels. Historically, the river was a broad series of braided channels, and flood events would have changed the shape and location of those channels frequently – though I don’t know how important ice jams were relative to annual high flows from Rocky Mountain meltwater.
I have a few predictions about how this year’s flood will impact prairies, but they are just educated guesses….
The grasslands that were covered by water for a few weeks will get a boost in their soil moisture for this upcoming season. We’ve had a relatively dry winter, so that soil moisture should create some very different growing conditions for plants in flooded versus non-flooded prairies. Depending upon a number of variables, flooded areas might also retain more standing water in sloughs, creating valuable habitat for many wildlife species.
Floodwaters carried grass and other debris into fences and pushed them right over.
The flooding carried more than just large chunks of ice out of the river and through the prairies. It also picked up and carried downed trees and branches, along with other assorted buoyant objects, natural and man-made. In addition, the ice and water scraped vegetation from some places and deposited it elsewhere. We burned a portion of prairie last year and left it ungrazed as a seed harvest site, so it was covered in tall big bluestem and other grasses. The ice essentially shaved some of that area as it came through, scraping away most of the standing vegetation and leaving behind a site that now looks as if it had been intensively grazed. That shaved off grass was deposited further downstream along obstacles (such as fences) and in high spots as the water levels dropped. As a result, some areas of prairie are now covered by a foot or so of fairly dense thatch. It will be fun to watch how that thatch affects vegetative growth and wildlife use.
This area was burned last spring and ungrazed. Before the flood, it was a stand of tall dense big bluestem and other tall grasses. The ice and water essentially shaved the tall stems and leaves off and carried them away.
This Canada goose decoy appeared to view its new surroundings with equanimity (look it up, Dillon) after being deposited by receding flood waters.
A second decoy looked a little less at ease.
This layer of flood-deposited thatch is about a foot thick. It will be interesting to see what impact it has on vegetation beneath it.
I’m guessing most of the impacts of this flood will be positive, or at least interesting, from a land management standpoint. One exception is that we’ll have some fairly major fence repair to do this spring. A bigger concern is a potential influx of invasive plants carried in to our prairies as seed or vegetative material from the river. Phragmites, reed canarygrass, and purple loosestrife are probably the most likely and potentially serious invaders, but others such as Canada thistle, salt cedar, and Russian olive are also possible. We’ll need to be vigilant over the next few seasons to make sure we catch new populations of those invaders before they can become well-established.
Ice fields like this make it difficult to travel across the prairie and get a full picture of what was flooded and what wasn’t.
Once the remaining ice melts away, we’ll get out and explore more of our flooded areas. After we have mapped out (at least generally) the boundaries of the flooded areas, we’ll watch and evaluate what impacts the ice and water actually have on the prairie this season and beyond.
And then we’ll see how good my predictions are…
The Prairie Ecologist 
The Potomac this morning on my bike ride in!!
Cheers
Tim
When hay mowing or firebreak mowing leave thick layers of thatch on prairie here this often results in the death of the smothered vegetation. Typically such spots are small, so no real problems seem to result. But if you have large areas thickly smothered . . . .?
I think Chris should burn the thatch off this spring.
I changed my mind. Chris should burn as much of it up as possible now. It is packed so densely it should burn. If he burns it up now he will reduce the chance of sterilizing the soil. If he waits until burn season the soil will probably end up getting sterilized.
James McGee Sir, you are a hard man to track down. I am a member of NARGS and I am interested in the plant of Sisyrinchium albidum that you presented on the back cover of The NARGS Journal Fall/Winter 2013 with your article on Sledges. It seems like a remarkable superior form and I would be interested in acquiring seeds / offsets of it. My main interest are native wildflowers and my retirement project is germinate native orchids from seeds. I have a few unique items that I could offer you in trade. Robert Swartz reswartz@hotmail.com
Robert, thanks for the comment. We’ll be watching closely. We have the option of burning off the thatch if that looks to be necessary, but it’s hard to know how big the patches are or how uniformly thick they are until the ice recedes more. At this point, I think we’ll just keep an eye on things and see if further action is warranted. The voles should be happy!
Although leaving the thatch would kill all plants, it would also create some interesting microclimates. The thatch is a good insulator and therefore will not conduct heat away. During the summer in the sun the surface of the thatch may reach over 150 degrees F. You should watch this carefully because this is approaching the temperature where manure piles spontaneously combust. Another interesting result is the insulating qualities of the straw will keep the ground cool. The ground may not completely thaw until mid-summer. You might have an interesting micro-climate around the thatch covered areas where the sand draws up cool water from the melting ice under the thatch through capillary action. If you leave the thatch your neighbors should be thanking you. The result will be less water consumed by your prairie and more going into the ground for their crops.
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I like your positivity and enthusiasm in response to change, Chris. Some would see devastation and get depressed over all the work apparently for naught. While there will be repairs, you see this event as a natural process no less worthy of study and reflection as forms of active management. I think some scarred and thatchy areas need to be left as is to determine how the system responds to this form of disturbance, since there are not many chances to observe it any more given the changes in water abundance and management in the Platter River. I know you will keep us posted!
Like the drone photos! Gives a much better perspective of how expansive prairie is. Be interesting to have an article on drone s and drone photography.
What would stop you from burning the thach befor it reseeds the new location?
I was just wondering about the role of ice on ecology both terrestrial and aquatic. I even wrote a brief post from a Montana perspective. I really appreciate your exploration of possible impacts and look forward to how it plays out this summer (an potentially years to come).
Why is the river flooding now? Mountain run-off has not yet begun here in Colorado. Is it a result of the river icing over?
Misi, my impression is that it was just the effective damming of the river by ice, not particularly high flows.