Wharfe Walkers Jack Hanson and Patrick Godden, both 27, collected hundreds of kilos of rubbish from riverbanks along the way.
They also raised thousands of pounds to fund independent testing of the Wharfe at Ilkley - the first river in Britain to be given bathing water status.
Bathing water status, which was granted by the Government at the end of last year, means the water will be regularly tested to ensure it is clean and safe enough for swimmers.
Patrick said: "Many families paddle, play and swim there, but like most rivers in the UK, it is heavily polluted."
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Jack added: "The Wharfe isn’t the only UK river with these issues.
"We wanted to raise awareness across the country, so we walked to London via rivers like the Ouse, Trent and Thames, and lakes and reservoirs like Rutland Water, collecting rubbish as we went."
Patrick said: "The amount we collected was staggering, and it was a hard slog at times but our spirits were always lifted by the warmth and generosity of the people we met along the
way, like the café owners in East Yorkshire who set us up for the day with a free breakfast and the pub customers in Lincolnshire who had a whip-round for us."
The two men presented a letter to Environment Minister Rebecca Pow on arriving in London.
It read: “Our walk was an effort, as two self-funded 27 year-olds, to take action and inspire others to do their bit for the country that we love and that has given us so much.
"We have collected over 300 kilos of litter in less than two weeks, walked 265 miles, and raised £4,000 (so far) for independent testing of the Wharfe.
"One of us is a global strategic alliances manager for a major tech company and the other is a full-time student in his final year of study, proving that anyone can make a difference with enough will, determination… and blister plasters.
The letter went on: “We are proud of what we have achieved but know that to markedly accelerate the pace of change we need ministers like yourselves to do as we have done; to take risks, to be brave and inspire others to pass crucial legislation that will bring Britain to a leadership position on the world stage.
"It’s time to walk the walk."
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