Riparian condition assessment for the Pawnee River (version 2.0), Fort Larned National Historic Site, Kansas

Martin, Michael; Wagner, Joel

The National Park Service (NPS) is preparing a Natural Resource Condition Assessment for Fort Larned National Historic Site. To inform this assessment, the NPS Water Resources Division was asked to evaluate the functional condition of the Pawnee River riparian area within and adjacent to the park. We performed our assessment during a June 2013 site visit. Using methods described in “A User Guide to Assessing the Proper Functioning Condition and the Supporting Science for Lotic Areas” (U.S. Department of the Interior, 1998), we determined that the entire reach of the Pawnee River at Fort Larned is in a “nonfunctional” condition (lowest of three possible ratings). This rating is given to riparian areas that lack adequate vegetation or channel/floodplain forms necessary to dissipate energy during flood flows. As a result, they may become unstable during flood events and exhibit excessive erosion or sedimentation, with resulting loss of many of the beneficial functions and values provided by healthy riparian ecosystems. The nonfunctional rating assigned to the Pawnee River riparian area at FOLS is primarily due to a lack of connection between the stream channel and the local water table for any appreciable amount of time during the growing season. This is most likely related to a regional drop in the water table due to decades of groundwater withdrawal. Groundwater levels in the vicinity of FOLS have declined anywhere from about 10 to 20 feet since the mid – 20th century, and while they have apparently stabilized, they are still well below pre-development levels (Balleau Groundwater, Inc., 2006). In addition to the water table decline, the channel has aggraded about 10 feet above its base level due to a concrete check dam just downstream of the park boundary. The sediment that has deposited upstream of the dam since it was constructed exacerbates the problem of disconnection between the channel and the water table. The most dramatic effect of this disconnection is the almost complete lack of perennial riparian-wetland vegetation in the channel bed and on its lower banks. Despite the nonfunctional rating, we don’t believe that the Pawnee River is likely to become destabilized by low to moderate-frequency floods in the near-term because the check dam provides substantial, albeit artificial, grade control. However, if the dam does fail in the future, or it is removed without careful sediment management, the channel bed would incise to obtain a new, stable grade and a great deal of sediment would be discharged downstream. Additionally, the channel banks at FOLS could become de-stabilized if the channel adjusts laterally to the new hydraulic conditions, possibly threatening park infrastructure. Ultimately, such channel adjustments could result in a more functional riparian system, especially if they reestablish a connection between the channel bed/lower banks and the water table. But, the time it would take the river to “re-stabilize” is not predictable, and the degree of potential improvement in riparian condition is uncertain. A focused study of water table conditions in the Pawnee River riparian area at FOLS would help the NPS understand the potential for river restoration and achievement of a properly functioning riparian ecosystem.

Riparian condition assessment for the Pawnee River (version 2.0), Fort Larned National Historic Site, Kansas

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