That confused me a bit based on what I remembered about trout. A quick check confirms my memory that the colder the water the greater the concentration of dissolved oxygen.
https://www.usgs.gov/media/images/temperature-affects-dissolved-oxygen-concentrations
This is wicked intriguing. I don’t care what the truth is, because everybody is working overtime. I haven’t been on the water without a GPS for many moons. I expect a certain base speed for the warmup stage, about 30 minutes for about two miles. If you go out at the same time three days in a row with the same wind speed/direction, each day will be different conditions. Ignore the fact that consecutive days of exertion deplete energy stores unless you reload within the glycogen window. Just considering tide. Tide flow ratchets up about 45 minutes each day. There’s usually two highs and two lows. But each day, they go from the 1st being a high-high. The next high phase is a low-high. Next day, the 1st high is a medium-high as will the 2nd high be a medium-high. Day 3 will start with a low’high, paired with a high-high. You can substitute term like spring and neap tide. I’m not sure if its right, but the basic principle is right.
To make a long story longer. Salt and fresh water have different densities. I don’t want to go there other than to say one flows on top of the other. If the Piermont region is inundated with rain, it flows to the ocean’s and increasing volume of fresh water fights to find a level in competition with incoming salt water. One layer increases in depth and the velocity of each level changes as friction causes swells or chop. Those forces churn up nutrients through the layers. Salinity isn’t uniformly distributed either. The earth’s rotation flings heavier water to one side of the bay.
If wind comes from one direction and dies off then builds the same direction then dies, or if it veers or goes variable, the impact is finite. On the other hand if its moderate but constant over the course of several days, a 10 mph wind can build a .6 mph current in half a day. A longer run or stronger wind can build monsters. Then a .6 mph outflow butts heads, or joins, or cross winds come into the picture, or a river empties into the flow, everything gets comical.
Another twist for a shore, basin or river is how wind loads it. A south wind loads the Chesapeake Bay, a north wind empties it. A noreaster pushes water in, low/high bsrametric pressure, astrological anomalies with 3 ft over normal which might be 2 ft at a high high in the spring or 1.2 ft in the fall.
If it’s the same person, same boat. Same location. My guess is it could influence an increase in speed at a different level or cause more mixing due to friction. I use this as an example. If you go on a lake, you might think its easier, until you ask a local what’s it like to paddle lake michigan because there aren’t any currents - ha ha ha ha! Or San Franciso Bay must be like the Chesapeake Bay. I don’t remember where the trip happened. If it was in the ocean, I’m not sure what the current does there. If it was Florida, let me know. I want to paddle there. By the way seaweed here is a pain when the tide drops. As the boat goes into low water or seaweed, it seems to create some added resistance
Hope nobody wanted to ask what’s a good first boat. I don’t know. Not sure if that helps. The principal is sound, even if my numbers are iffy. Good luck with that. If somebody sees errors. Please let me know.
Never claimed that water density might change. Just expressed doubts the the density change would be significant enough over a short period of time to affect the speed of a kayak by 15%.
rival 51 is correct that as the temperature of water decreases it can hold more dissolved gases. Heating a carbonated beverage will cause it to go flat.
I was a biochemist.
Yes Rival 51 is correct, I will admit it. Some weekends I put my brain in backwards.
It is possible that such a thing can happen, anything can happen; but it is probably something else.
If the algal load is high enough ( possible with so much rain flooding septic systems and washing fertilizer out of yards) the Dissolved oxygen can be be deleted at might and skyrocket. That would reduce buoyancy and create drag. I still don’t know how to measure a 15% addition to drag.
Thanks all for the great conversation. It isn’t the current since it is a fresh water lake.
I never did figure out what happened but it happened again. It was probably a combination of oily “sticky” water after the storm with lots of plant material in the water and the fact that I used my all carbon Epic mid wing that day and not my usual Epic small mid.
Aside from current, the only thing I notice when paddling is how shallow water slows the boat considerably, but you would know that from paddle depth. The other thing is seaweed, even if not visible on top, but you would feel that as well with the paddle.
Have your paddles been vaccinated?
As far as water density and temperature, there is only about a 4% decrease in density from 32°F to 212°F.
It is probably just a higher amount of suspended or dissolved solids in the water. They will cause more friction against your hull as you try to push through it.
Florida sand does tend to get washed into streams and rivers as part of the runoff.
It’s easy to measure the density of water. Density of fresh water is mass/volume. Temperature does have a small effect. Values can be found Here. Simply take a sample of this viscous water and measure it and compare it to a day when you are 15% faster.
Another inexpensive option is to purchase a hydrometer to measure the density directly.
You can also measure the viscosity here.
Unless the water is approaching the viscosity and density of mud I doubt that you will find a reason that the water itself can account for a 15% decrease in measured speed. Water depth, however can cause a marked decrease in speed. Are you paddling over the exact same areas when you notice this 15% decrease in speed?
These measurements of density and viscosity usually represent what will cause a difference in the low single digits at the extremes. Sounds more like a problem with your propulsive engine on various days if kayaking on the same route and with the exact same conditions.
Side tangent here. I keep my reef aquarium at an SG of 1.026. Ergo ocean water is 2.6% heavier by volume than distilled. So if you were in ocean water, your boat would displace 2.6% less volume (but same weight) than in fresh which would reduce surface area and make you go slightly faster. I wonder how fast I could paddle in the Dead Sea. 
Or would more solid create more drag.
Or it could have been a large school of flatulent fish swimming just ahead of you, causing a 5% increase in surface turbulence and aeration leading to a 3% decrease in buoyancy, which lead to a 6% increase in wetted surface,…
Turbulent flow.
Holy mackerel. That would be scary.
