RIVERcurrents, unlike tidal currents, flow ceaselessly toward the sea. However, where the rivers empty into the sea, they are influenced by the rise and fall of the tides, sometimes for great distances inland. Here, the “Rule of Twelfths” does not apply (see Tides and Tidal Currents). Rising tides and flooding currents slow the river current or even reverse it, forcing water to flow upriver. Likewise as the tide falls, ebb currents add to the force of the river current. Some kayakers deliberately seek the challenges and thrills of this combined current in places where the shoreline constricts it into boiling whitewater. How much influence the tide has over the river current depends upon such factors as the strength and volume of the river current, the tidal range, and the geography of the river. COPYRIGHT 2002 DIAMOND PASS PUBLISHING Kayakers can use these currents to their advantage when going with them, or they can seek out back-currents or eddies close to the river banks when the current is adverse. Beware, however, of standing waves, which are often formed during the ebb, when the combined river and ebb currents are opposed by a wind blowing upriver. COPYRIGHT 2002 DIAMOND PASS PUBLISHING Other dangers lurk in especially narrow constrictions. For example, Upper and Lower Hells Gate, between the Kennebec and Sheepscot River, squeeze the currents to six or eight knots. Rocky bottoms redirect the current upwards into upwellings or boils that fan out at the surface—often with enough force to send a small boat rocketing sideways—and the eddyline can be pocked with whirlpools.