The primary concerns in the Yahara Watershed are sedimentation, excess nutrient loading leading to algae and aquatic plant growth, decreased water clarity, stream channelization (straightening), and streambank erosion. Other issues included low dissolved oxygen concentrations and thermal loading issues from urbanized areas.
Different computer modeling efforts have produced various loading estimates for sources of nutrients and sediment. For example, Montgomery Associates (2011) suggested through the use of the SWAT model that agricultural sources accounted for 90% of sediment loads and 84% of phosphorus loadings and urban sources provided 10% of loading for sediment and 14% of phosphorus. In contrast, the priority watershed project (1993-2010) suggested greater contributions from urban sources (up to 30%), particularly construction sites.
Lathrop (2007) analyzed phosphorus loadings to the lake from 1980–2006. One of his important findings was that snowmelt and “drizzle-day” runoff events during the January to March seasonal period constituted 43-48% of the subwatersheds’ long-term phosphorus load through 2006. Not only is this load significantly import to the overall input, the phosphorus is much higher in its dissolved form compared to runoff events during warmer months when sediment concentrations are higher.
Lathrop and Carpenter (2011) recommend that the overall phosphorus load to Lake Mendota should be reduced by 50% in order to improve the quality of Lake Mendota and the downstream lakes. This is consistent with the recommendation of the priority watershed project (WDNR 2000), and with the analysis by Montgomery and Associates (2011). While estimates and quantification of loadings to the tributaries and Lake Mendota may vary somewhat, all of the studies are consistent in their conclusions: the water quality in the watershed is being compromised by excessive phosphorus and sedimentation, and management efforts to reduce the loadings will result in improved water quality.