Yes it is possible. Maybe do a SphereCast in the direction of the flowing water, for any colliders in that cast that have a rigidbody, add force. I like this idea better than an invisible collider and OnTriggerEnter, but those are 2 methods.

http://docs.unity3d.com/Documentation/ScriptReference/Physics.SphereCast.html

http://docs.unity3d.com/Documentation/ScriptReference/Collider.OnTriggerEnter.html

EDIT : I have made a script from the following Unity Script References :

http://docs.unity3d.com/Documentation/ScriptReference/Physics.RaycastAll.html

http://docs.unity3d.com/Documentation/ScriptReference/Physics.SphereCastAll.html

castDirection is a world-space Vector3; meaning if you put (-1,0,0), then the direction is pointing negatively along the X-axis; put (0,1,0) and the direction running positively along the Y-axis(pointing up). Try Different values for x,y and z, just keep them between -1 and 1.

I have now included a linedraw-er in the script, you can see what is affected by changing these values. You shouldn't include this in your build, it's just handy for showing where the spherecast is in the editor while designing and building.

castRadius, castDistance and amountOfForce should be self-explanatory =]

EDIT 2 : this script has been edited to include a time for the flow to be active and then time for the flow to be inactive. Set the flowTime (currently 10seconds on, 10seconds off) :

#pragma strict

private var castDirection : Vector3;
private var castRadius : float;
private var castDistance : float;
private var amountOfForce : int;
private var flowCounter : float = 0.0;
private var isFlowing : boolean = true;
private var flowTime : float = 10.0;

function Start()
{
    // build direction vector * Note : this is in World-Space
    castDirection = Vector3( -1, 0, 0 );

    // Radius of SphereCast
    castRadius = 3.0;

    // Distance of SphereCast
    castDistance = 9.5;

    // How much force to push on the rigidbody's
    amountOfForce = 100;
}

function Update() 
{
    flowCounter += Time.deltaTime;

    if (flowCounter < flowTime)
    {
        isFlowing = true;
    }

    if (flowCounter > flowTime)
    {
        isFlowing = false;
    }

    if (flowCounter > (flowTime * 2.0))
    {
        isFlowing = true;
        flowCounter = 0.0;
    }
}

function FixedUpdate() 
{
    if (isFlowing)
    {
        // Show SphereCast in editor (this is optional)
        ShowSphereCast();

        // create an array to store all rayhits
        var hits : RaycastHit[];

        // Physics.SphereCastAll( position : Vector3, radius : float, direction : Vector3, distance : float );
        hits = Physics.SphereCastAll( transform.position, castRadius, castDirection, castDistance );

        for (var i = 0;i < hits.Length; i++) {

            var hit : RaycastHit = hits[i];

            Debug.Log("hit " + hit.collider.transform.gameObject.name);

            if ( hit.collider.transform.gameObject.rigidbody != null )
            {
                hit.collider.transform.gameObject.rigidbody.AddForce( castDirection * amountOfForce * Time.deltaTime );
            }
        }
    }
}

function ShowSphereCast() // (this is optional)
{
    Debug.DrawRay( transform.position, castDirection * castDistance, Color.cyan );
    var origin : Vector3 = transform.position;
    var segments : int = 50;
    var calcAngle : float = 0;
    var posX : float[] = new float[segments + 1];
    var posY : float[] = new float[segments + 1];
    // Calculate Arc
    for (var i:int = 0; i < segments + 1; i ++)
    {
        posX[i] = ( Mathf.Sin( calcAngle * Mathf.Deg2Rad ) * castRadius ); //  + ( i * curvage );
        posY[i] = Mathf.Cos( calcAngle * Mathf.Deg2Rad ) * castRadius;
        calcAngle += 360.0 / parseFloat(segments);
        // Show outside of SphereCast
        Debug.DrawRay( Vector3( 0, posY[i], 0 - posX[i] ) + origin, castDirection * castDistance, Color.red );
        // Show origin of SphereCast
        Debug.DrawLine( Vector3( 0, posY[i], 0 - posX[i] ) + origin, origin, Color.red );
    }
}

I have a web build of an example to show it works. The above script is on an empty gameObject, just under the white pipe =]

wasd or arrows to move green sphere : rigidbodystream webbuild

Everything is possible, you just need to know how :)