Lenhart Interviewed by CSA News
A closer look: stream geomorphology and ecohydrological interactions in the Minnesota River Basin
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CSA News: The June CSA News story pointed out how complex this topic is; can you briefly describe how your research fits into ongoing research on the Minnesota River basin?
Lenhart: The research I have been involved with shows there have been a lot of changes to the stream geomorphology, both from direct channelization and other modifications, especially in the headwater streams. This has increased the transport of sediment and nutrients downstream and makes the channels more entrenched.1 In the lower Minnesota, we also see a loss of channel length and an increase in river width, which tends to indicate an increase in sediment and nutrient loading. We are finding this has occurred since 1938, so the last 70 years. So while there is a lot of discussion about bluff erosion being the main source of sediment, it appears that the stream banks are also a major source, at least in the past century.2
Another part of my research looks at the ecohydrological interactions, in particular, the relationship between channel width and riparian vegetation. We are finding that changes to the duration and the timing of flow, especially in the summer, have reduced the growth and establishment of riparian vegetation in the Lower Minnesota River.2 Because of this, the channel isn’t able to narrow through vegetation encroachment on the point bars. We looked back at some of the old maps, from 1855, compared them with 1938, and you can see that there was actually a narrowing of the channel in the 1930s. This is not surprising, as research on other rivers has shown that during times of low flow and drought, the riparian vegetation encroaches on the channel. But in the last 30 to 40 years, the water has been high all the time, and we only observe the channel getting wider.
Photo provided by Christian Lenhart: Minnesota River at high flow near Bloomington, MN.