Complex sentence: Unlike other larger estuaries, the Columbia river supplies relatively little nitrate to the estuary, rather the primary source of nitrate to the estuary is from ocean exchange caused by wind driven up-welling off the Oregon and Washington Coasts.
Split into simpler sentences: Unlike other larger estuaries, the Columbia river supplies relatively little nitrate to the estuary. Rather the primary source of nitrate to the estuary is from ocean exchange caused by wind driven up-welling off the Oregon and Washington Coasts.