Agenda 11-02-23; 4-a - Resolution Endorsing Consensus Principles II for Revised Falls Lake Rules

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�n=30 During wetter rainfall periods, high nutrient loading from the watershed associated with high precipitation and stream flows can transport higher sediment and nutrient loads to Falls Lake. These events may increase flushing and reduce residence times. Figure 4 shows the Flat River nearly rising to the bridge deck following a high rainfall event. Periods of low loading from the watershed and to small inflows and longer Figure 4.Flat River rising near the bridge deck following a UNRBA correspond g High Flow Sampling Storm Event residence times when algae are provided longer periods for growth. Sporadic storm events that occur during periods of long detention can provide pulses of nutrients to Falls Lake that may remain stagnant in the lake for an extended time. The complex relationship between hydrologic condition, nutrient loading, and lake residence time means there is not a predictable cause-and-effect relationship between loading and algal growth. Figure 5 shows that in upper Falls Lake (upstream of Interstate 85), chlorophyll-a concentrations have been declining since the 1980s. This part of the lake historically had more variability in chlorophyll-a concentrations as demonstrated by the longer boxes and whiskers in Figure 5. Chlorophyll-a data were not collected in the upper lake between 2008 and 2012. Since 250- upper data collection restarted in 2013, 200- 150 - chlorophyll-a concentrations have 100 0 been relatively stable. The middle 50 part of the lake (between Interstate J ° M Idd le 85 and Highway 50) has lower ab 250 " chlorophyll-a concentrations and 200X50 less variability than the upper lake. a 100 Chlorophyll-a concentrations in this o 50 $7 d�—_ ��, $* Eli — segment have been relatively (� Lower stable since 2001. In the lower 250- 200- part 5020apart of the lake (downstream of 150 Highway 50), chlorophyll-a 100 concentrations have been relatively 50 stable over the life of the reservoir, IT 0 ID °� ��""�""°{�' ° �M C N� �r��"C °M rn rn!c rn rn rn m rn m rn T rn rn o 0 0 0 0 0 0©c 0 0 0 0 0 0 0 0 even in the 1980s when TT TTT concentrations in the upper lake Figure 5.Distribution of Chlorophyll-a Measurements in Upper Falls Lake (upstream of Interstate 85),Middle Falls Lake(between Interstate 85 and were much higher. During the Highway 50)and Lower Falls Lake(downstream of Highway 50 to the Dam) UNRBA 2014 to 2018 monitoring period, chlorophyll-a concentrations in Falls Lake were highest in 2017 (especially below Highway 50) compared to other UNRBA monitoring years, but nutrient loads that year were approximately half(see the following discussion of UNRBA Watershed Modeling for more information on this characteristic). 11