The lake is no longer stratified. This is in contrast to 11/6/96 when the majority of the lake was strongly stratified with virtually anoxic conditions existing in the deeper parts of the hypolimnion (CAWCD data). One area of the lake that was not stratified during 11/6/96 was the area behind the towers. This could be due to this area being mixed by wind and wave action as it is close to shore and more shallow than the rest of the lake, or turbulence created by release/input from the towers. The lake turned over during 11/6/96 - 12/4/96 possibly due to decreased temperatures in the epilimnion causing it to increase in density until it met or exceeded the density of the hypolimnion. The anoxic conditions that occur in the hypolimnion during summer thermal stratification in most lakes are a result of high epilimnetic biomass that "rains" through the metalimnion and thermocline becoming trapped in the hypolimnion. This may lead to increased BOD in the hypolimnion and, therefore, an oxygen deficit. This oxygen deficit may have profound impacts on water chemistry and composition. If the sediment is exposed to prolonged periods of anoxia, reducing conditions may prevail. This may lead to the formation of sapropel. Sapropel is high in H2S and CH4 and has a shiny black color due to the presence of ferrous sulfide. This has been anecdotally referred to as the "rotten-egg" odor associated with high levels of discharge from the hypolimnion into the Wadell forebay. During the fall overturn, large amounts of sediment may be disturbed and entrained in the outlet ports of the towers. Nutrients, such as nitrogen and phosphorous species will become unbound from their normally close association with iron, aluminum, and calcium under anoxic conditions. These dissolved nutrients become concentrated in the hypolimnion as the "organic rain" from the epilimnion continues to add material that can become reduced in the hypolimnion.

During the sampling trip of 12/4/96, water from the CAP was being backflushed into the lake. This was water that had remained inactive in the entire 150 mile west-end of the canal while work was being completed on the Centennial Siphon. Approximately 3000cfs from Lake Havasu was being used to "push" the inactive canal water into Lake Pleasant while the Phoenix area was utilizing water from canal storage (memo from Tim Kacerak CAWCD). We sampled from 3 sites; at the towers, behind the old dam, and approximately 1000 yards N. of the towers. Samples at depth from the tower location proved difficult due to the inability to maintain the boats position because of the high rate of discharge from the towers. We obtained samples for nutrient analysis (orthophosphorous as phosphate, nitrate as nitrogen, and total phosphorous as phosphate), and algal enumeration and identification.

Algae counts showed much higher numbers at depth than at the surface for all 3 sites sampled (see Old Dam algae counts, 1000 yards algae count, or Tower algae count). The trend appeared to be that the deeper the sample was taken from, the higher the numbers. This correlates with the nutrient analysis. The total phosphorous was higher where the numbers of algae were higher while the orthophosphorous and nitrates were, generally, lower where the numbers of algae were higher (see Old Dam nutrients, 1000 yards nutrients, or Tower nutrients). The counts were markedly higher at depth the further you were away from the towers. One possible explanation for this is that the incoming water from the canal was colder (having less thermal mass to maintain a stable temperature than the lake water). This would cause it to be more dense than the lake water and sink to the bottom entraining the algal cells with it. The turbulent conditions would cause some of it to mix, initially, with the lake water. As the turbulence decreased as you moved away from the towers the density gradient became greater causing a smaller, more dense gradient as you moved away from the intake.

The consequences of this may be significant. As previously stated, the lake is not currently stratified. The dissolved oxygen readings produce a more or less orthograde line so there appears to be adequate oxygen at the sediment/water interface. As the algae dies some of the nutrients released may chelate with the oxidized iron, aluminum, or calcium in the sediment. Upon summer thermal stratification, however, when reducing conditions prevail in the hypolimnion the sediment will release these nutrients possibly causing a more nutrient-enriched hypolimnion than previous summers. If water is released from the hypolimnion during this time it may add to nutrient enrichment of the canal and further eutrophication. Canals (and flowing water in general) need far fewer nutrients to produce large amounts of algae than does standing water due to the fact that algae in flowing environments always has a renewed supply of nutrients. Therefore, even small increases in the nutrient levels within the canal may lead to a large degree of eutrophication.

More studies would be needed to determine exactly what happens to the incoming water from the canals into the lake, what happens to it once it's in the lake, and what happens to it after it is discharged into the canal from the lake. Possible projects include tracer studies of the nitrogen and/or phosphorous through this cycle to determine where it goes and what happens to it, sediment analysis to determine the contents of the varves and determine it's chelating ability with nutrients throughout the year, or simply more intensive analysis at several more sites with the hydrolab sonde while collecting samples at depth for nutrient/algae analysis.