Riou entered the navy at 12 years of age, and after a period spent in British and North American waters, served as a midshipman on Captain James Cook's third and final voyage of discovery.

After nine weeks at sea, and with continued labour and endurance, Riou successfully navigated his half-sunk ship back to port, saving the lives of those who had elected to remain with him.

[3][5] The expedition was Cook's third voyage of discovery, and after his death at Hawaii Clerke took command, transferring to Resolution and bringing Midshipman Riou with him.

[6][9] Discharging from the Ganges in June 1784, he went on to half-pay, which lasted for two years until he received another appointment, this time to the 50-gun HMS Salisbury in March 1786.

[6] A further period on half-pay followed his discharge from the Salisbury in November 1788, but meanwhile he had succeeded in attracting the attention of the Townsend family, and was able to use their patronage to secure an appointment to command HMS Guardian, in April 1789.

[6][8][9][10] The Guardian was a former two-decked 44-gun fifth-rate, but had been armed en flûte and loaded with stores to be taken to the British colony at Botany Bay.

[6][8][9] In addition to these stores, consisting of seeds, plants, farm machinery and livestock with a total value of some £70,000, the Guardian was also to transport a number of convicts and their overseers.

[6][8] With over 300 people aboard his ship, Riou left Spithead on 8 September 1789, and had an uneventful voyage to the Cape of Good Hope where he loaded more livestock and plants.

[7] Bligh had arrived at the Cape from Timor, where he had landed after a 3,618 mile voyage in an open boat following a mutiny aboard his ship, HMS Bounty.

[8] After completing his re-provisioning, Riou sailed from the Cape in mid-December, and picking up the Westerlies, began the second leg of his voyage to New South Wales.

[8] On Christmas Eve, twelve days after his departure from the Cape, a large iceberg was spotted, and Riou decided to use the ice to replenish his stocks of fresh water that were quickly being depleted by the need to supply the plants and animals he was transporting.

[11] Riou ordered the stores, guns and livestock to be thrown overboard in an attempt to lighten the ship, but was injured when his hand was crushed by a falling cask while trying to clear the bread-room.

[11] Riou at last agreed to this, well aware that there were not enough boats for everyone, and announced 'As for me, I have determined to remain in the ship, and shall endeavour to make my presence useful as long as there is any occasion for it.

'[12] While the boats were prepared, Riou wrote a letter to the Secretary to the Admiralty; Sir,If any part of the officers or crew of the Guardian should ever survive to get home, I have only to say their conduct after the fatal stroke against an island of ice was admirable and wonderful in everything that related to their duties considered either as private men or on his Majesty's Service.As there seems no possibility of my remaining many hours in this world, I beg leave to recommend to the consideration of the Admiralty a sister who if my conduct or service should be found deserving any memory their favour might be shown to her together with a widowed mother.I am Sir remaining with great respectYour ever Obedt & humble servt,E.

[13] Realising that this was providing extra buoyancy, Riou had the gun deck hatches sealed and caulked, while another sail was sent under the hull to control the flooding.

Having now created a substitute hull out of his deck, Riou raised what little sail he could and began the long journey back to land, with the pumps being continuously manned.

[10][13] The Cape of Good Hope was sighted on 21 February 1790, and whalers were despatched from Table Bay to help the battered ship to safety.

[6][9] Appointed to command the sixth-rate HMS Rose in June 1793, he served in the West Indies with Sir John Jervis and was active in the operations against Guadeloupe and Martinique in 1794.

[9][14] He was moved to the 40-gun HMS Beaulieu in November 1794, capturing a number of small French vessels before ill-health forced him to be invalided home.

[6][9][16] He was active against French privateers, before being assigned to Sir Hyde Parker's expedition to the Baltic in 1801 to compel the Danes to abandon the League of Armed Neutrality.

As Nelson's ships of the line engaged their Danish counterparts, Riou took his frigates in to harass the Tre Kroner forts and blockships.

[19][20] Lieutenant-Colonel William Stuart, commanding the soldiers of the 48th Regiment recorded that Riou: ...was sitting on a gun, was encouraging his men, and had been wounded in the head by a splinter.