WASHINGTON >> Just before dawn one summer day in Washington, the president of the United States stripped naked on a rock by the river, plunged in and saw a dead man float to the surface.

We know this about John Quincy Adams because he kept a diary for the ages. So have many presidents, from George Washington to Joe Biden. In these journals — a collection of notebooks in Biden’s case — they confide to themselves, express raw opinions, trace even the humdrum habits of their day and offer seat-of-the-pants insight on monumental decisions of their time.

Here, also, they may possess and spill secrets they shouldn’t. That’s part of why Biden is facing more congressional scrutiny this week for his sloppy handling of classified documents after his vice presidency. Meantime Donald Trump became the first person in history to be charged with a crime for making off with sensitive government records as president — and then, unlike Biden, resisting demands to return them.

Adams called his diary his “second conscience” — not to mention a place to record his frequent skinny-dipping in the Potomac — and presidents since have vouched for the value of scribbling down the day’s observations or dictating them to a recorder to help them think things through and preserve them in memory, if not memoirs.

“The process of converting a jumble of thoughts into coherent sentences makes you ask tougher questions,” Barack Obama said of his journaling.

Jimmy Carter, who came away from the White House with more than 5,000 pages of transcribed entries, allowed, “I seldom exercised any restraint on what I dictated.”

Dwight Eisenhower wrote in a diary entry not only about infighting on a scientific advisory panel, but its highly secret (if questionable) analysis that Soviet atomic bombs could be rendered 99% ineffective by surrounding them with a type of radioactivity to which they were uniquely vulnerable.

Trump’s diary was named Twitter.

Now Robert Hur, the special counsel who probed Biden’s treatment of classified material but declined to recommend charges, is to appear before a House committee Tuesday to explain findings that left both parties ambivalent, for opposite reasons.

Democrats are relieved Biden won’t be charged but upset that Hur cited old-age memory fog as one reason the president should get a pass. Republicans wanted Biden prosecuted yet were delighted to see Hur fuel the public’s sense that Biden is too old for the job.

The hearing is bound to touch on the ample history of presidents who left office with documents containing state secrets, even after the 1978 Presidential Records Act mandated that the government has “complete ownership, possession, and control” of all presidential and vice presidential records. The act was just one part of a reformist clean-up of government from Richard Nixon’s corrupt presidency.

A president’s diary is exempt from that act, at least when its content does not relate to the conduct of official business, but classified information is not supposed to be there.

According to Hur’s report, Biden’s diaries contained highly classified reflections on foreign adversaries, homeland threats and notes from the President’s Daily Brief, including some determined to be “top secret” with markings signifying they came from human intelligence sources — among the most closely controlled secrets in the U.S. government.

He appeared to keep multiple sets of notes, one organized for his daily reflections and another devoted to foreign policy. The papers gather information explaining why he disapproved of President Obama’s plan for a U.S. troop surge in Afghanistan.

“There is evidence that, after his vice presidency, Mr. Biden willfully retained marked classified documents about Afghanistan and unmarked classified handwritten notes in his notebooks, both of which he stored in unsecured places in his home,” Hur wrote.

“He had no legal authority to do so,” Hur went on, and the president’s actions “risked serious damage to America’s national security.” But he said the evidence falls far short of proving that Mr. Biden retained and disclosed these classified materials willfully.

The special counsel investigation drew a sharp contrast between Biden and Trump, crediting the Democrat with fully cooperating in the return of documents he shouldn’t have had, consenting to searches in several places and submitting to an interview that lasted more than five hours.

“After being given multiple chances to return classified documents and avoid prosecution, Mr. Trump allegedly did the opposite,” Hur said. “According to the indictment, he not only refused to return the documents for many months, but he also obstructed justice by enlisting others to destroy evidence and then to lie about it.”

Presidents who have been less conspicuous in disdaining the rules have not faced such trouble.

Ronald Reagan, Hur noted in his report, left the White House in 1989 with eight years of handwritten diaries, “which he appears to have kept at his California home even though they contained Top Secret information.” The Justice Department took no known steps to retrieve or secure the diaries.

Carter dictated entries to his diary and had them typed by a secretary. Returning to Plains, Georgia, after his presidency, Carter realized he had 21 large volumes of double-spaced text, excerpts of which became a book.

Hur said there is “some reason to think” Carter and another enthusiastic diarist, George H.W. Bush, both had classified information in their diaries. But that was hardly shocking.

“Historically, after leaving office, many former presidents and vice presidents have knowingly taken home sensitive materials related to national security from their administrations without being charged with crimes,” Hur wrote.

Students of history have always placed great stock in the unguarded musings of high officials, whether in a diary, a leaked conversation or one of the thousands of White House recordings that six presidents secretly taped from the Franklin Roosevelt administration to the downfall of Nixon.

Those episodes are rarer in the scripted, polished and controlled enterprise of the modern American presidency, under any recent president not named Trump.

They “provide unique windows into the presidency, helping us better understand how policy is made and power is used,” said Marc Selverstone, director of Presidential Studies and co-chair of the Presidential Recordings Program at the University of Virginia’s Miller Center. “You get it, as LBJ would have said, with the bark off.”

To be sure, there wasn’t much guarded about John Quincy Adams, judging by the 15,000 pages of diary entries he wrote over more than 68 years, four of them in the White House. His diaries “comprise the longest continuous record of any American of the time,” says the Massachusetts Historical Society, which publishes them online.

On May 26, 1828, Adams closed a long, detailed post about his “harassing day (of) crowded and multifarious business” with happy news from his garden. “I perceived a tamarind heaving up the earth,” he wrote, and he planted Hautboy Strawberries.

Another day, he enjoyed “sitting naked, basking on the bank at the margin of the river” after a swim. No secrets there.

On July 22, 1825, his early morning routine of walking and swimming took a dark twist.

“I walked as usual to my ordinary bathing place, and came to the rock where I leave my clothes a few minutes before sunrise — I found several persons there, besides three or four who were bathing; and at the shore under the tree a boat with four men in it, and a drag net … in search of a dead body.”

“I stripped and went in to the river; I had been not more than ten minutes swimming when the drag boat started, and they were not five minutes from the shore when the body floated immediately opposite the rock; less than one hundred yards from the shore.”

Thus the diary of the sixth president notes the death of Mr. Shoemaker, a post office clerk seen swimming about in the water until he was gone.

AP White House Correspondent Zeke Miller in Wilmington, Delaware, contributed to this report.