I'm inclined to believe it's the temperature rise rather than the UV, otherwise the top 6" of every stream and lake would be sterile.

I lifted up and noticed some submerged coyote turds about one foot away. I suffered no harmful effects whatever.

Perhaps the UV sterilized the shallow rocky potholes from which I used to drink in Arizona. I vividly recall one into which I plunged my face and drank deeply. Finally satiated, I lifted up and noticed some submerged coyote turds about one foot away. I suffered no harmful effects whatever.

Perhaps the UV sterilized the shallow rocky potholes from which I used to drink in Arizona. I vividly recall one into which I plunged my face and drank deeply. Finally satiated, I lifted up and noticed some submerged coyote turds about one foot away. I suffered no harmful effects whatever.

the mortality of the microorganisms increases when they are exposed to both temperature and UV-A light at the same time.

Water temperature affects disinfection

A warmer water temperature speeds up the process. However, bacteria, viruses, giardia and cryptosporidia are killed by UV-A radiation even when water is cooler.

* UV-A interferes directly with the metabolism and destroys cell structures of bacteria.
* UV-A (wavelength 320-400 nm) reacts with oxygen dissolved in the water and produces highly reactive forms of oxygen (oxygen free radicals and hydrogen peroxides), that are believed to also damage pathogens.
* Infrared radiation heats the water. If the water temperatures rises above 50°C, the disinfection process is three times faster.

At a water temperature of about 30°C (86°F), a threshold solar radiation intensity of at least 500 W/m2 (all spectral light) is required for about 5 hours for SODIS to be efficient. This dose contains energy of 555 Wh/m2 in the range of UV-A and violet light, 350 nm-450 nm, corresponding to about 6 hours of mid-latitude (European) midday summer sunshine.

At water temperatures higher than 45°C (113°F), synergistic effects of UV radiation and temperature further enhance the disinfection efficiency.

Thanks EMP, this is something new to me.
From reading in the SODIS links you supplied it seems they suggest getting the water above 50°C to deal any amoebic dysentery (if present).

7. Is temperature important?
Short Answer

Yes and no. SODIS works much faster at temperatures above 45°C, but in most circumstances the water does not get this hot.
Full Answer

SODIS has been found to work faster above 45°C, and even three times as fast when the water exceeds 50°C (Berney et al). However, it is rare for ambient temperatures to get this high, and because SODIS bottles do not absorb much infrared radiation (which is the form of light that causes an increase in temperature) the water does not usually reach this threshold. For these reasons it is uncommon for this temperature factor to play a role in actual SODIS treatment.

The Water School has done a number of tests in Kisoro, Uganda, which is a mountainous area. In all of these tests the water temperature never approached 50°C, yet 100% kill of E. coli (the standard test organism) was observed. The highest temperature was 43°C and in most cases it was in the low 30's. The important thing is that SODIS is effective at the temperatures we experience in this area. It can be even more effective at higher temperatures, but does not impact our current projects.

Some have recommended SODIS bottles should be painted black on one side to increase the heat absorption from sunlight. Our experience does not support this recommendation. It adds another step and detracts from sustainability. This issue is addressed more fully in Q&A 10.
References

Berney et al., Efficacy of solar disinfection of Escherichia coli, Shigella Flexner, Salmonella Typhimurium and Vibrio cholera. J Appl Microbiol. (2006).

I assume amoebas would generally be present if there was fecal contamination, or am I wrong about that?

Some organisms such as Enteroviruses, Cryptosporidium,
Giardia and Amoebae however are more resistant than E.coli.
The absence of E.coli therefore does not necessarily indicate
their removal. Spores of sul?te-reducing Clostridia can be
used as an indicator for these organisms [11]. But such
analytical methods cannot be used for routine tests under
?eld conditions as they are time-consuming and expensive.