An Adaptive Management Approach for Summer Water Level Reductions on the Upper Mississippi River System

Abstract

The primary purpose of this report is to provide an adaptive management approach for learning more about summer water level reductions (drawdowns) as a management tool, including where and how drawdowns can be applied most effectively within the Upper Mississippi River System. The report reviews previous drawdowns conducted within the system and provides specific recommendations for learning more about the lesser known effects of drawdowns and how the outcomes can be influenced by different implementation strategies and local conditions. The knowledge gained can be used by managers to determine how best to implement drawdowns in different parts of the UMRS to help achieve management goals. The information and recommendations contained in the report are derived from results of previous drawdown projects, insights from regional disciplinary experts, and the experience of the authors in experimental design, modeling, and monitoring. Modeling is a critical part of adaptive management and can involve conceptual models, simulation models, and empirical models. In this report we present conceptual models that express current understanding regarding functioning of the UMRS as related to drawdowns and highlight interactions among key ecological components of the system. The models were developed within the constraints of drawdown timing, magnitude (depth), and spatial differences in effects (longitudinal and lateral) with attention to ecological processes affected by drawdowns. With input from regional experts we focused on the responses of vegetation, fish, mussels, other invertebrates, and birds. The conceptual models reflect current understanding about relations and interactions among system components, the expected strength of those interactions, potential responses of system components to drawdowns, likelihood of the response occurring, and key uncertainties that limit our ability to make accurate predictions of effects (Table 1, Fig. 4-10). Based on this current understanding, the main questions still associated with drawdowns include (1) the effects of frequency of drawdowns (from once every few years to multiple years in succession); (2) timing of the beginning of drawdowns (follow the descending arm of the flood pulse versus always beginning in early summer); (3) long-term benefits (greater than 5-6 years), especially as compared to known short-term loses (e.g., mortality of mussels in exposed areas, loss of submersed vegetation in exposed areas, cost of advanced dredging); and (4) the effects in northern (above pool 14) versus southern pools (pool 14 and below, and the Illinois River). An adaptive management design should address these questions to reduce uncertainty in predictions of drawdown effects and help determine if different implementation strategies are needed in different parts of the system. Given that drawdowns will continue to be used as a management tool on the UMRS, we suggest that some drawdowns be conducted in an adaptive management context that helps meet management objectives, but also provides efficient learning about the questions listed above. We propose two different, but interrelated, experimental designs to address these questions. Both designs call for conducting multiple drawdowns in multiple pools (2-4 pools) to allow direct comparison of results and produce rapid learning. However, the report does not provide a detailed scope of work for carrying out the designs. If managers choose to implement one of the experimental designs, specifics of choosing appropriate pools and developing a monitoring plan will need to be determined through collaboration among managers, researchers, and statisticians. We suggest characteristics to consider in selecting treatment and reference pools (study sites) and also provide guidance for developing a monitoring plan. Some aspects of these two designs could be implemented individually, but by implementing individual elements, direct comparisons of some design features