For days they have been marching to Capitol Hill from every corner of the country, pilgrims in an annual rite of spring, intoning chants of Assiniboine, Navasota, Brazos, Souris and other sweet river names.

They are businessmen, preachers, lawyers, governors, mayors, housewives, environmentalists, farmers, and other pillars who have one aim in mind: petitioning their Congress.

This is the time of year when they troop before the House and Senate Appropriations committees to plead for or against the flood control and water-resource projects that are as much a part of Americana as apple pie.

Hour after hour for the last several weeks, the energy and water resources subcommittees have taken testimony from hundreds of pro and con witnesses as they work to develop the fiscal 1981 appropriation.

But there are so many projects, so many points of view, so many witnesses who want time before a subcommittee that these hearing rooms take on the air of factory assembly lines.

Witnesses are allocated only a few minutes each to have their say. Senators and congressmen, only sparsely present, listen with a kind of perfunctory detachment that suggests they have heard it all before.

In a way, they have.

For example, there was a round of applause in the crowded Senate subcommittee room for H. K. Thatcher. He was here for the 40th year, plugging for projects in the Ouachita Valley of Arkansas, saying pretty much the same thing.

They have heard it in other ways, as well.

Legislators who ordinarily decry excessive federal spending have convenient lapses of restraint when their own districts' projects hang in the balance.

Tuesday was Texas Day at the subcommittees -- witnesses appear in the House and Senate on the same days to help them save time and money.The lapses of restraint were Texas-sized.

"I want everything in the budget and I want a little bit more," fiscally conservative Rep. Jack Brooks (D-Tex.) told the Senate subcommittee.

"Pork-barrel is a valid term for negative cost-benefit projects," said fiscally conservative Sen. Lloyd Bentsen (D-Tex.). "But these projects are clearly important to my state . . . These are investments in the future."

Fiscally conservative Sen. John Tower (R-Tex.) was at an Armed Services Committee hearing "fighting for more defense money" as an aide put it, but he sent along a water statement.

"I know we meet during a time of deep budgetary concern. It is time for controlling runaway inflation and balancing the federal budget," Tower's statement read. But a full level of federal spending is necessary for Texas water projects, he said.

As in everything else, however, inflation is eating away at the constitutional right to petition Congress and it even is affecting the Texans.

"A lot of these witness groups are trying to cut back their costs," said Proctor Jones, longtime Senate subcommittee assistant. "It is a long way to Washington and it is very expensive to come. Texas used to send 250 people on chartered planes. This year they only have between 55 and 70 people in their delegation."

There are, of course, witnesses who come in more austere ways, from longer distances. The record probably was set this week by Gerry McKinney, who rode 43 hours on a Greyhound bus from Winnipeg, Canada, to speak against a U.S. water project.

McKinney sat patiently at the rear of the hearing room and then, as the last witness Tuesday, was given a chance to talk about Canadian objections to the Garrison Diversion Project in North Dakota.

Jones said McKinney's appearance was the first he could remember by a foreign national coming before the subcommittee to testify on a U.S. project.

McKinney was representing Manitoba farmers, tourism leaders, fishermen and Indians who fear that completion of the Garrison project will ruin their province's environment and water resources.

Garrison involves a canal and reservoir system that would link the U.S. Missouri River with the Souris River in Canada to provide an irrigation system for North Dakota farmers.

In the process, however, Canadians fear, and an international boundary commission agrees, that Canada's Hudson Bay watershed would be seriously damaged if now-stalled Garrison is completed.

McKinney and fellow witnesses from North Dakota noted that completion of Garrison would violate a 1909 U.S. treaty with Canada. The Canadian government has registered protests on the matter.

A little to his surprise, McKinney was allowed to testify and Sen. Harrison Schmitt (r-N.M.), sitting as chairman heard him out.

"I do not believe that this highest elective body in your land would deliberately take an action that would so damage the citizens of my home province," said farmer McKinney.

McKinney proudly flashed his bus ticket, which carried a "Merci, Canada" seal -- a reference to Canadian assistance in helping six Americans flee Iran. "I thought I'd come on a thank-you pass. And I'd like to go back in that same spirit," he said.