COLD RIVER DIVING BELOW A WARM LAKE

Warm water concentrates fish, cold water scatters fish.

In the dog days of summer trout have a limited options to find comfortable temperatures, oxygen and food. They will venture into warm water briefly to feed. As anglers we are looking for the conditions that hold trout.

In addition to the thermocline there is a unique circumstance in the summer that creates edges to fish, where cold water meets warm,. To understand it you need to know that water is the heaviest, most dense at 40 degrees. As it gets colder it expands as it approaches freezing. It also gets lighter/expands as it heats up. The bottom of deep water lakes will be about 40 degrees. This article is about a major difference in the water temperature/weight of a cold river flowing into a warm lake.

Where I live on the west slope of the Sierra Nevada Mountains, there are a number of rivers that have multiple dams. On the Yuba River, Bullard's Bar Reservoir at 2000' elevation feeds, down to Englebright Lake 900' and then the water continues down the Lower Yuba River.

When Bullard's Bar was built, provisions were made to draft water from many depths to be able to choose from the temperatures available in the lake. In the spring, rice farmers want the warmest water to aid in sprouting their seeds. Throughout the hot weather months fisheries interests have made provisions for the coldest water from Bullard's Bar to cool the flows in the lower Yuba for the salmon and steelhead populations.

Another factor is the afternoon hydro-electric power demand for our air conditioners when valley temps reach triple digits. Bullard's Bar is the storage facility for water. Englebright is used as a flow meter for the lower Yuba. Englebright rises and falls on a 24 hour cycle, varying at most a half dozen feet. Flows from Bullards Bar into Englebright vary widely over a 24 hour cycle based on power demand. Outflows from Englebright are constant over a daily cycle.

The point of this article is the effect these factors have on Englebright Lake during heat spells in the summer. The daily flow cycle can vary but the flows from a recent mid-July week can be typical. Flows in the early AM hours can be as low as 150 cfs. As power demand comes on in midday, the flows are ramped up to over 3000 cfs for peak afternoon demand and from 6:00 pm through midnight the flows were around 1500 cfs.

On a recent trip the incoming water at the top of Englebright was 48 degrees. The surface water temperature close to the dam was 75 to 80.

It is the intersection of high volumes of cold water meeting a warm body of still water that is of interest to anglers. The colder/heavier river inflows dive below the warm lake forming a subsurface river in the depths. How to fish this subsurface flow is a topic for another day. But the spot where the cold drops below the warm can be a good feeding spot for a variety of fish.

The sign of this location is a raft of debris on the surface. As the afternoon flow increases it picks up pine needles, leaves, twigs and all sorts of floating material. These items ride the cold current until it dives below the warm lake water. Here the floating debris forms a raft on the surface. On a recent trip the water surface temp from one side of the debris raft to the other went from 75 degrees to 48 in less than 100 feet. The air felt like a warm summer evening below the debris raft and you almost needed a jacket on the cold water side above. Note: Englebright Lake is a flooded narrow canyon that confines this phenomenon between steep walls. A river flowing into a broad side of a lake will not be as confined and easy to see.

This stark change is most pronounced when the flows are high, 3000 cfs. Into the evening as the flows are cut the debris raft breaks up a bit and scatters with the breeze. There is a reliable up-canyon breeze each afternoon in the summer. The temperature change becomes more gradual and the 75 to 48 degree shift can be over hundreds of yards rather than just a few feet.

Typical feeding behavior for fish in rivers is to find still water where they can rest, adjacent to moving water which carries food. Another thing I have noticed higher up above the lake in the river water, is the fish starting to actively feed as the water is dramatically raised in a short period of time. The heavy currents put more food into the water and the trout are conditioned to this as the dinner bell.

Many of these factors come together on warm summer afternoons on Englebright Lake. Is it a guarantee of fish in the boat? Not necessarily. But it is a good place to look. This phenomenon is not unique to Englebright Lake. These same conditions can be found on river systems in the Sierra and other places in the West. Use your thermometer and/or debris rafts to find sharp temperature breaks. It just may put you onto the fish.