Effects of Tidally Varying River Flow on Entrainment of Juvenile Salmon into Sutter and Steamboat Sloughs

Abstract

Survival of juvenile salmonids in the Sacramento–San Joaquin Delta (Delta) varies by migration route, and thus the proportion of fish that use each route affects overall survival through the Delta. Understanding factors that drive routing at channel junctions along the Sacramento River is therefore critical to devising management strategies that maximize survival. Here, we examine entrainment of acoustically tagged juvenile Chinook Salmon into Sutter and Steamboat sloughs from the Sacramento River. Because these sloughs divert fish away from the downstream entrances of the Delta Cross Channel and Georgiana Slough (where fish access the low-survival region of the interior Delta), management actions to increase fish entrainment into Sutter and Steamboat sloughs are being investigated to increase through-Delta survival. Previous studies suggest that fish generally “go with the flow”—as net flow into a divergence increases, the proportion of fish that enter that divergence correspondingly increases. However, complex tidal hydrodynamics at sub-daily time-scales may be decoupled from net flow. Therefore, we modeled routing of acoustic tagged juvenile salmon as a function of tidally varying hydrodynamic data, which was collected using temporary gaging stations deployed between March and May of 2014. Our results indicate that discharge, the proportion of flow that entered the slough, and the rate of change of flow were good predictors of an individual’s probability of being entrained. In addition, interactions between discharge and the proportion of flow revealed a non-linear relationship between flow and entrainment probability. We found that the highest proportions of fish are likely to be entrained into Steamboat Slough and Sutter Slough on the ascending and descending limbs of the tidal cycle, when flow changes from positive to negative. Our findings characterize how patterns of entrainment vary with tidal flow fluctuations, providing information critical for understanding the potential effect of management actions (e.g., fish guidance structures) to modify routing probabilities at this location.

Flow Characteristics at a River Diversion Juncture and Implications for Juvenile Salmon Entrainment

“…For example, fish telemetry tracking was conducted by Steele et al [19] along the Fremont [3]. Field data at the Sutter and Steamboat junctures of the Sacramento [32] and the linear regression developed by [31] based on the fish telemetry data at seven flow junctions of the Sacramento River are also plotted.…”

“…Figure 8 also includes the linear regression line based on the fish telemetry data measured at seven flow junctions on the Sacramento River (developed by [31]). Additionally, the fish tracking data measured by Romine et al [32] at the Sutter and Steamboat junctures of the Sacramento River are also plotted. Discussion regarding this is offered later.…”

“…Studies for fish entrainment at river junctures have mostly been performed in the field through tracking. The measured data are often used to develop empirical relations that correlate fish entrainment to flow variables, e.g., the flow diversion ratio [13,14,[30][31][32]. There is a lack of numerical modeling studies to my knowledge.…”

Swimming behavior of emigrating Chinook Salmon smolts

“…At the landscape or basin scale, behavior generally cannot be observed directly, but is inferred from large scale movement of animals, such as in [ 22 ] where timing of emigration was linked to basin hydrology. Behavior at the scale of river junctions, and the associated route selection, is of great practical interest and has been successfully related to local hydrodynamic forcing in studies such as [ 23 , 24 ]. In particular [ 24 ], correlated lateral position of fish (relative to the critical streakline) to route selection 250 m downstream, which can be seen as a test of the persistence of cross-sectional distributions.…”

Development and Evaluation of a Chinook Salmon Smolt Swimming Behavior Model

“…While routing into Old River increases the entrainment risk, routing down the San Joaquin River is Acoustic telemetry data are useful in understanding pathways of emigrating Chinook salmon through the Delta and studies have shown that mortality varies with migration route [6,7]. Acoustic telemetry has also shown that fish are not homogeneously distributed in channels and therefore the proportion of fish entering a specific channel at a divergent junction (diffluence) can be different from the proportion of flow entering that channel [8]. This difference indicates the importance of swimming.…”