1. Introduction

Since the late 1800s, human intervention has significantly altered the structure of Elkhorn Slough, and the circulation within it. Before 1946, Elkhorn Slough (ES) was a sluggish backwater with little influence from Monterey Bay. In 1946, the Army Corps of Engineers created Moss Landing Harbor located at the head of Monterey Submarine Canyon by cutting through the dune barrier that separated the Slough from Monterey Bay. ES is now a shallow, tidally-forced embayment that is directly coupled to Monterey Bay. Increased tidal action has led to a dramatic increase in tidal scouring, particularly in the lower part of the Slough [1]. The Slough was transformed from a fresh water, brackish environment to one that is primarily saltwater, and from an estuary that was primarily depositional to one that is now dominated by erosion. Since 1971, maximum tidal currents in the main channel at the mouth have increased from approximately 75 to 150 cm/s. The flood tide introduces relatively clear water from Monterey Bay while waters discharged during the ebb are laden with sediment eroded from the banks and bottom of the

Slough. This discharge creates a plume that extends 3 km or more offshore and is clearly visible in satellite imagery [2]. As erosion continues, the tidal prism and currents in ES increase, leading to further erosion. This behavior suggests a system with positive feedback, a situation that can become unstable.

Recent estimates of the erosional sediment losses from the Slough have ranged from 3.5 <sup>10</sup><sup>4</sup> <sup>m</sup><sup>3</sup> /year [3] to 8 <sup>10</sup><sup>4</sup> <sup>m</sup><sup>3</sup> /year [4]. Observations of bank erosion indicate average losses of 40 cm/year [5]. Bank erosion has widened the main channel, and the resulting increased tidal prism has led to an expanding network of tidal creeks that feed into the Slough (John Oliver, personal communication). According to Malzone and Kvitek [6], the tidal prism increased by 43% during the last decade and the surface area covered by salt water increased by 48%. Based on aerial surveys, losses to the surrounding wetlands (composed primarily of Salicornia marsh) have increased significantly over the past 50 years [7]. Erosion has led to an overall loss in vegetated marsh around the Slough that was most apparent during the first decade following the opening of Moss Landing Harbor.

Many environmental factors such as loss of Salicornia marsh, ichthyoplankton abundance, nutrient distributions, phytoplankton distributions, pesticide accumulation, and changes in biodiversity are related directly, or indirectly, to the circulation and physical properties of ES. According to Dyer [8], the physical, chemical, biological, and geological systems within a given estuary are, to a large extent, inseparable. Less is known about the physical environment of ES than perhaps some of its other defining properties. This is due in part to the spatial and temporal complexities of the circulation, and the rate at which physical changes to the system are occurring.

It is our purpose to summarize what is known about the tides, the circulation, and the physical properties of ES, based on a period of data collection that began in 1970 and continued into the early 2000s. In this regard, the manuscript is a period piece that spans a recent 30-year period. The work contained herein is based on the work of the first author's students, M.S. theses by some of these students, and, importantly, new observations. The text covers (1) the physical setting of the Slough, (2) the tides, (3), the physical properties, and finally, (4) the conclusions. This manuscript is based primarily on information that was contained in an unpublished report by Broenkow and Breaker [9]. However, a number of significant refinements have been made to that report which are included here for the first time.
