Equilibrium Response of Riparian Vegetation to Flow Regulation in the Platte River
W. Carter Johnson, Department of Horticulture, Forestry, Landscape, and Parks, South Dakota State University, Brookings, South Dakota, 57007, (605) 688-4729 (4713), JOHNSONC@mg.sdstate.edu
The Platte River historically responded to water development by rapid channel narrowing and expansion of native riparian habitat. Riparian woodland (Populus-Salix) expanded most rapidly in the 1930's and 1950's. Since the 1960's, open channel and woodland area in most reaches have remained relatively stable. This apparent equilibrium has been maintained despite relatively low mean and peak flows during the past decade. The maturation of woodland vegetation may have contributed to the open channel/woodland equilibrium. The "middle-aged" woodlands which now dominate the Platte River floodplain are on higher surfaces, have higher-angled banks, and have less protective vegetation than at younger stages, all resulting in greater potential for erosion. Dis-equilibrium (i.e., woodland increases) has occurred in the past decade near Grand Island below areas where vegetation has been removed to increase channel area for cranes and other water birds. Mechanical clearing may have liberated sediment, locally aggraded the channel downstream, and stimulated tree and shrub recruitment. While cleared areas have more open channel area, downstream areas have less, suggesting that clearing may not be a river-wide solution to the channel narrowing problem.
The Myth of Platte River Water Depletion
Frank Kwapnioski, Acting Water Resources Manager, and E. J Dekleva, Jr., Nebraska Public Power and Irrigation District, P.O. Box 310, North Platte, NE 69103, (308) 532-9200 (4512), fskwapn@nppd.com, ejdekle@nppd.com
The purpose of this study was to effectively update the G.P. Williams study pertaining to a historical flow reduction in the Platte River. The Williams study was published in 1978, and this report incorporates recent data into the Williams study while adjusting methodology and examining alternative conclusions.
According to G.P. Williams, in "The Case of the Shrinking Channels - the North Platte and Platte Rivers in Nebraska" - USGS (1978), "Seventy percent of the Platte River's water has been depleted by agriculture and other human uses." This statement has been printed as fact in hundreds of daily newspaper stories, editorials, newsletters from environmental interests - even National Geographic Magazine.
Multiple flaws in William's assessment of Platte River water depletion have become evident. Selective use of data and inappropriate selection of representative gauging stations contribute to the inaccuracies in the Williams study. Additionally, data from the 1978-present time frame indicates that the Williams study was premature. The incorporation of more recent data, while using the same methodology as Williams, indicates that the mean annual flow, at the stations of interest, has not continued the same trend as that which occurred in the 1950's, as development has continued. Additionally, peak annual flows and associated trends were included in this study. This updated information brings into question the apparent correlations among development, river depletion and any correlation to vegetative encroachments. This study also incorporates recent finds relative to current trends in vegetative encroachment.
This study will develop and explain evident deficiencies in the Williams study. Additionally, this study will incorporate more recent data to assess whether determinations made by Williams were appropriate and timely.
Bridges and River Stabilization in the Big Bend Region of the Platte River in South Central Nebraska
A. Steele Becker, Department of Geography and Earth Science, University of Nebraska at Kearney, Kearney, Nebraska 68849, (308) 865-8355 (8980), becker@platte.unk.edu
The purpose of this research was to investigate the relationship between bridge occurrence and river behavior in an 88 mile reach of the Platte River from Lexington to Grand Island, Nebraska. It was triggered by two detailed mapping efforts of the reach in the early 1980's, which showed an apparent relationship between bridge occurrence and appearance of river channels, and more than 25 years of investigation of this reach of the Platte River. Research began with the expansion of a detailed linear data base from aerial photos, developed in the mid 1980s to investigate the unusually high flows in the summers of 1983 and 1984. Results of this original study, published in 1988, suggested bridges had a controlling effect on reaches of the river.
For the current study the data base was expanded to cover the period 1938 to 1988. A gradual change in the river from west to east involving channel gain and island loss was identified as well as three separate reaches of the river, based upon differences in gain/loss rates.
The data was then subjected to computer analysis and development of 3D graphics to reveal behavior patterns of the river. Separate analysis of the affect of the high-flow years of 1983 and 1984 was also included. Within the three separate river reaches, 13 bridge sites were randomly selected and subjected to detailed data collecting and statistical and computer analysis to determine affect on river behavior. Results of the computer analysis are displayed in 3D graphic form.
Results of the study clearly show that while the river system (channels/islands) have declined over the 50 year period for the study reach as a whole, with stability of the system established more than 20 years ago, the pattern of decline and change from west to east has not been consistent. Between Lexington and the I-80 Minden interchange island area exceeds channel area but from the Minden interchange to Grand Island channel area equals or exceeds island area, with the channel dominance increasing from the Shelton I-80 interchange eastward to Grand Island. Bridge occurrence dominates the river in the reach where channel dominance over islands occurs and stability is greatest. In this reach of the river, bridges and their frequency of occurrence are the primary controlling variable of river behavior.
Return to 1997 Platte River Basin Ecosystem Symposium
Last updated by Darren A. Jack on 2/6/97