There are several USGS gages on the Penobscot River.
In additional, the National Weather Services' "Advanced Hydrological Prediction" Services provides the following forecasts:
The USGS gauge at Eddington provides the best guidance for those paddling the stretch of the river between Old Town and Brewer, which has the most difficult rapids:
Less than 3': Low water. Rapids may be scratchy, but are less likely to swamp open boats. Careful route finding may be necessary.
3 to 7 feet: Medium Water: Best flows for clean lines, but may be challenging for loaded boat. Canoes should be outfitted with flotation. Portaging may be appropriate, especially early season.
Greater than 7 feet: High water. Long swims possible.