"True, after a few years, the most radioactive species decay, reducing much of the thermal output."

Caesium-137:
"Small amounts of Cs-134 and Cs-137 were released into the environment during nuclear weapon tests and some nuclear accidents, most notably the Chernobyl disaster. As of 2005, Cs-137 is the principal source of radiation in the zone of alienation around the Chernobyl power plant. Together with caesium-134, iodine-131, and strontium-90, it was among the isotopes with greatest health impact distributed by the reactor explosion."
http://en.wikipedia.org/wiki/Caesium-137

Chernobyl disaster:
"Chernobyl", was the worst nuclear power plant accident in history and the only instance so far of level 7 on the International Nuclear Event Scale, resulting in a severe nuclear meltdown.
...
The 2005 report prepared by the Chernobyl Forum, led by the International Atomic Energy Agency (IAEA) and World Health Organization (WHO), attributed 56 direct deaths (47 accident workers, and nine children with thyroid cancer), and estimated that there may be 4,000 extra deaths due to cancer among the approximately 600,000 most highly exposed and 5,000 among the 6 million living nearby."
http://en.wikipedia.org/wiki/Chernobyl_disaster

Caesium-137,caesium-134, iodine-131, and strontium-90 are the most lethal to humans because the human body will absorb them:

"Caesium-137 is water-soluble and extremely toxic in minute amounts. Once released into the environment, it remains present for many years as its radiological half-life is 30.23 years. It can cause cancer 10, 20 or 30 years from the time of ingestion, inhalation or absorption providing sufficient material enters the body"

Caesium-134 has a half-life of 2.0652 years

Iodine-131 (131I), also called radioiodine, is a radioisotope of iodine which has medical and pharmaceutical uses.
...131I decays with a half-life of 8.0197 days with beta and gamma emissions
Iodine in food is absorbed by the body and preferentially concentrated in the thyroid where it is needed for the functioning of that gland. When 131I is present in high levels in the environment from radioactive fallout, it can be absorbed through contaminated food, and will also accumulate in the thyroid. As it decays, it may cause damage to the thyroid. The primary risk from exposure to high levels of 131I is the chance occurrence of radiogenic thyroid cancer in later life. Other risks include the possibility of non-cancerous growths and thyroiditis."

Strontium-90 (90Sr) is a radioactive isotope of strontium, with a half life of 28.8 years.....
90Sr finds extensive use in medicine and industry, as a radioactive source for thickness gauges and for superficial radiotherapy of some cancers. Controlled amounts of 90Sr and 89Sr can be used in treatment of bone cancer.
...
Together with cesium isotopes 134Cs, 137Cs, and iodine isotope 131I it was between the most important isotopes regarding health impacts after the Chernobyl disaster. Slightly elevated levels of 90Sr may be present in the vicinity of nuclear power plants.

Strontium has biochemical behavior similar to calcium. After entering the organism, most often by ingestion with contaminated food or water, about 70-80% of the dose gets excreted. Virtually all remaining strontium is deposited in bones and bone marrow, with the remaining 1% remaining in blood and soft tissues. Its presence in bones can cause bone cancer, cancer of nearby tissues, and leukemia."
[All the above are cross linked from wiki reference: http://en.wikipedia.org/wiki/Caesium-137 ]

I would surmise that what causes most heat in spent fuel rods are Iodine-131 and Strontium ["90Sr is a product of nuclear fission. It is present in significant amount in spent nuclear fuel"]. And Iodine-131 because it has such a short half life {half-life of 8.0197 days}.

So in regards to your above statement:
"True, after a few years, the most radioactive species decay, reducing much of the thermal output."

I would say that only in regard to Iodine-131 would a "few years" be accurate. And in a couple decades one would also have almost 1/2 as much of the other dangerous radioactive and heat generating elements- though of course still quite potent and potentially lethal.