If there is great meaning in small events, then the humble people of the Rio Grande Valley, speaking in their native Spanish, may have touched it yesterday in the lobby of the posh Mayflower Hotel.

THere, light years from their dirt street and waterless Texas communities, they found an assistant secretary of agriculture who not only listened, but talked to them in their language.

They told Alex Mercure, one of the highest rank Hispanics in the Carter Administration, of their need for help in developing their own portable water supplies.

Mercure indicated that federal assistance would be forthcoming. That pleased Fidel Velazquez and Amado Garza, who are, respectively, "presidente" and "secretario" of El Comite Para Mejoramiento de Cameron Park.

The Cameron Park Betterment Committee, representing about 175 Mexican-American families in the village outside of Brownsville, Tex., is a tiny element of a larger political development.

Government has begun to listen to the voices of the estimated 12 million Mexican-Americans and other Spanish-speaking communities which are expected to be, by the turn of the century, the country's largest minority.

In the urban barrios and the dusty byways of the Southwest, the Hispanics are organizing, learning to talk the language of the grant-approving federal government, finding the political pressure points that provide help.

Variously, the House and Senate bills increase, rather than decrease, the federal share on dredging projects, waive negative benefit-cost ratios in some cases, and authorizes projects not yet studied by the Army Corps of Engineers.

A series of resolutions and hearings in the House and the Senate is seen as another gambit to reshape or flatly overturn Carter's plan for a more consistent, more cost-conscious national water resouces program

Notwithstanding the suppose public revulsion, toward more federal spending, waste and bureaucracy building, Congress seems to have gone out of its way to draw a wide line in the sand in front of Carter.

So as a milestone of sort, the National Council of La Raza (NCLR), one of the country's largest Hispanic organizations, is holding its first convention this week. In Washington, at the posh Mayflower.

The council, formed 10 years ago in the Southwest, now represents more than 100 community-based organizations that provide services to upwards of 1 million Hispanics. They counsel, they educate, they build.

Several hundred delegates from these local organizations are here for the speeches, the seminars an the official actions that attend most conventions.

But beyond that, as council president Raul Yzaguirre explained it, there is another point:

"We are here in Washington because we wanted to make an impact on the federal government. We are here and we are going to continue to be here."

Politically, Yzaguirre conceded, the council still has a distance to travel. Invitations to President Carter and Vice President Mondale to address the convention were rejected.

"We are very disappointed that the president failed to be here, at the largest or organization representating Hispanics, or at any of the other Hispanics groups that have asked him to speak," he said.

"This administration raised a lot of expectations for Hispanics that have not been fulfilled. The Carter administration has done better, to some degree, than its predecessors. But the standard ought to be their commitment."

To prod the administration a bit, the council's board of directors over the weekend urged the president to convene a White House Conference on Hispanics as soon as possible to deal with subjects ranging from immigration policy and bilingual education to an accurate count of Hispanics in the 1980s census.

For now, the administration's response to the council was to trot out most of the high-ranking Hispanics who came to Washington with Carter and make them available as seminar speakers.

Keynoting it, however, was not a Hispanic, but a black - Patricia Roberts Harris, the secretary of housing and urban development - who took the occasion to announce several HUD grants for Hispanic housing efforts.

Before doing so she urged La Raza's council to pull together, just as black civil rights groups did before the Latinos got organized, to use their political muscle.

She warned against "a new kind of prejudice" symbolized by the "acceptance of a black Cabinet member but the rejection of a subsidized housing project for the poor . . . a kind of prejudice born of the false allegation that people are poor only because they choose to be so."

That was a message understood by Beatriz Chapa, who comes from the hamlet of Del Mar Heights, 12 miles north of Brownsville in the Rio Grande Valley.