there is no order, or such a thing as a width or height pixel. not sure what you mean by this. each pixel in the image stores a depth. so you have  307200 values each storing a depth. very simple. 

What I mean to ask is how to get the depth of pixel x,y. in other words, what is the index of pixel x,y in the depth array? I'm trying to iterate through all the depth values and map them to a matrix, which is why their order in the array (and thus the order in which I traverse the array) matters. 

you lost me. the value IS the depth. super simple. not sure what you mean when you say index? depth array? what matrix? please explain precisely otherwise i  can't help you. also code example?

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you maybe know the algorithm to find an index of a pixel when we tknow his x and y coordinates (images processing tutorial)
loc = x + y * width;
but IS/ map depth map scenemap are 1d arrays
 the first index (index[0] starts at 0,0) the 2nd at (1,0) 3rd (2,0) etc etc
it also exist an algorithm to find x and y coordinates of a pixels which we know is 1D index ( his index in dmap smap ismap)

dmap = context.depthMap();
for(int i = 0 ; i< dmap.length ; i++){
x = loc[i]%width; y=(loc[i]-loc[i]%width)/ width; }

but manipulate arrays with this size( width*height values) in a dynamic process (draw()) is very onerous with Java.
Java is not the best language to solve it, i suggest you c++

Never mind about the matrix/mapping thing - that is unrelated to my question. I know that each value in the array is the depth. What I'm looking for is a way to get the index of the depth at pixel x,y. 

No. As sachaamm pointed out, the formula is: int index = x + y * width;

See almost every Processing sketch that uses the pixel array and: http://processing.org/learning/pixels/

Code Example

1. void setup() {
2.   size(640, 480);
3.   println("The number of pixels is: " + width * height);
4.   int index = 320 + 320 * width; // x + y * width;
5.   println("The index of pixel (320, 320) is: " + index);
6.   exit();
7. }