By Dr. David Wartinbee, for the Redoubt Reporter
Photo courtesy of David Wartinbee. Braided streams enter Skilak Lake from Skilak Glacier. In early spring the outflow is bluer, since colder temperatures limit glacial melt, and the amount of glacial flour in the water.
Our local glaciers and ice fields are no longer melting at the fast rates they were earlier in the summer. However, some have large lakes at their terminus that will continue to release water into the Kenai River drainage for some time to come.
Those glacier outlet streams are quite interesting to hydrologists and stream ecologists alike. When seen from the air, they have a distinctive and well-recognized morphology. These streams are usually heavily braided. This means there are all kinds of small branches from any one channel, and shortly downstream, that small channel meets with another or breaks off to form yet another channel. The channels collectively can look like a network or complex web of channels spread all across the valley floor.
The multiple channels of glacier outlet streams are caused by three basic conditions. The first condition is the abundance of smaller sediments being washed away from the glacier. These finer sediments are formed as the glacier grinds its way over underlying rock layers and pushes them to the front of the glacier. Because many of the sediments are fairly small, the water easily carries them away from the glacier. The finest particles, called glacial flour, will remain in suspension and cause the stream water to take on a milky, muddy or concrete-slurry color. If you have ever driven past the entrance to Denali National Park and crossed the gray Nenana River, you understand the concrete color description.
The second is the fairly high gradient, which enables water to move at a rapid velocity. This high water velocity causes erosion of underlying substrates as it moves along. That same stream velocity will cause some of the substrates to be carried in the water column or bounced along the stream bottom. Even with a high stream velocity, the stream cannot carry all of the materials, and some of it gets piled up along the edges of the channels.
The third situation that can play a role in forming braided streams is a frequent change in water discharge. Water discharges from glaciers will fluctuate with changing air temperatures. A warm day or a warm rain can suddenly increase the amount of water flowing away from a given glacier. On a cold day, like some this past week, when the freezing air temperatures halt ice melting, the outflow stream is reduced significantly.
With a quickly lowered amount of water in a stream, some of the channels will be dewatered, while others will be able to continue downcutting into the substrates. When the water discharge increases again, some channels will be re-watered, while others will end up remaining emptied. Suddenly the stream has a new set of active channels.
The erosional aspect of these glacier outflow streams causes the gradient to decrease as it cuts downward. A decreased gradient means the water slows. Slower water speed means the stream can carry less sediment materials in that particular area. With materials being dropped by the stream, it can start causing a slight damming effect. Backed-up waters will eventually cut through the deposited materials and rush downhill along yet another channel.
A meandering stream that suddenly has a large influx of fine sediments will almost immediately take on a braided-stream morphology. This was shown several years ago when there was severe flooding on the southern part of the Kenai Peninsula.
Huge amounts of water backed up at a small culvert for a small meandering stream called Silver Salmon Creek. Eventually, the water washed out a long section of Oilwell Road.
All the gravel sediments that had been used to elevate the road above the swampy valley bottom were suddenly carried downstream. Overnight, Silver Salmon Creek was choked with a massive amount of fine sediments and the stream created multiple channels through the area. Fortunately, when the proper-sized culvert was permanently installed and the road reopened, the engineers knew to move the sediments aside and reform the stream meanders that would normally have been there.
Glacier outflow streams are not good places for aquatic organisms. Because the substrates are constantly moving and being shifted around, only a small number of diatoms and algal cells will survive on the gravel. With very little to feed upon, there will be very few organisms able to eek out a living in these barren habitats. To make matters worse, the water temperatures of glacier outflow streams are very low, and this discourages other aquatic organisms.
Dr. Alexander Milner from University of Birmingham has been studying glacier outflow streams in Alaska for more than 25 years. In particular, he has been looking at how the streams and the resident aquatic organisms change as a glacier recedes uphill. He has found that a few midges, like Diamesa, are the first inhabitants, and they can endure the coldest waters and the worst substrate conditions. Only as the water temperatures rise and the stream substrates become more stable do other common aquatic insects like mayflies and caddisflies start to appear in the stream.
Because of the constantly changing substrates and the abundance of fine sediments, salmon and trout will not spawn in these streams. In some cases they will swim through the turbid and heavy sediment-carrying stream, but they will only spawn in more stable channels. Remember that salmon need to find established hyporheic zones so their eggs can survive.
Glacial outflow streams are common in Alaska and they present beautiful vistas when seen from above. However, because of the conditions that create their characteristic braided form, they are not often called home by aquatic insects or salmonids.
David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the ecology of the Kenai River and Cook Inlet watershed.
